Filtration Basics

By Graham Quick & Terry White

• Mechanical Filtration
• Biological Filtration
• Ready Made Filters
• Nitrate Filters
• Ultraviolet Systems

A filter system serves two purposes for the pond, the first is to act as a mechanical filter and the second is to act as a biological filter. Not all ponds will need filters but in most cases where fish are the priority they will be required in order to keep water clarity and toxic levels below the fishes tolerance. With any system built for a pond the results will differ from one to another, as there are so many factors and variables that can affect each pond. What works for one pond may not work for another; if in doubt seek advice from a reputable supplier. Do not buy the smallest and cheapest you can find; look around for the best product that will be easy to maintain and can run for a reasonable amount of time between cleaning, this is important for when you go on holiday.

Tip: Small compact filters will clog quickly, as they have less media than larger ones and will require more maintenance. Look out for labels that say low maintenance and easy clean, as many are not.

The filter is a very important part of the pond system and you should spend time getting the best you can afford. You will be cleaning it so the choice is whether you want to spend a lot of time cleaning a small filter or just the occasional clean of a larger filter.

Mechanical Filtration

There are many ideas as to the importance of the first stage of filtration, but it is the most important stage to get right for a number of reasons. The idea of the mechanical filter is to remove particles from suspension and in turn reduce the load on the biological section of the filter.

• If large quantities of waste get through to the biological section of the filter it will clog the filter media and reduce the filter's capacity to break down waste products.
• The more organic waste there is the more oxygen is required to break it down to harmless products. More oxygen demand means less for the fish in times of hardship, such as in hot weather.
• The quicker the large waste matter is removed the less work the biological filter has to do and the lower the nitrate level will be, therefore leading to less food for the algae, and less algae growth.
• If waste is kept to a minimum, the size of the biological filter can be smaller as it will have less work to do.

There are numerous methods for mechanical filtration, the most commonly used are a screen or media to remove or trap particles. The other method is to use a vortex.

The basic principle of the settlement tank is to slow the water down so that the suspended particles fall to the base of the tank, from where they can be removed with ease. In order to break up and slow the flow of water there has been a number of medias created to do this.

Filter brushes: Filter brushes are the most commonly used and is in fact the one used incorrectly in most installations. The brush should hang in the flow so that the water has to pass through the side, not from beneath or through the end. The water level should be below the top of the brush to stop the water bypassing the settlement system by flowing straight over the top of the brushes. The brush systems' biggest down fall is that it is very messy and time consuming to clean.

Bio-Block: Bio-Block is another media that is now available to the pond world. A simple media created by the joining of net tubes into ‘blocks’ which can be used in the filter, either whole or cut to fit. This media has many advantages over others, the most important is the easy maintenance and it is almost impossible to clog with flow rates of 7500 to 10000 litres per hour per m2 of surface area. Good if you go on holiday. The longer it is established the better it works as the bacteria build up a filter web between the net elements and trap the finer particles. Being easy to clean without removal makes this an excellent media for all ponds.

Vortex: The vortex is one of the most under used systems available, but many that are sold are poorly designed. The flow rate is very important with a vortex. The water enters the vortex at a 90º tangent to the side this causes the water to spin, creating a vortex (see picture below for basic design). At the correct flow the waste works its way to the centre of the tank and falls into a collecting chamber. If the flow is to fast the vortex will pick the waste up (like a tornado) and carry it on to the next stage of filtration, this is not the desired effect.

The exit of the vortex can also affect the efficiency of the unit. There are two trains of thought for this, the first is a centrally mounted pipe that is set just below the water level and collects the water from the centre of the water column (Exit 'A'), and the second (Exit 'B') is an outer trough that collects the surface water as it over flows into it from around the edge.

The ideal flow rate is between 12,000 to 17,500 litres per hour per square metre of surface area, this is for tanks over 1m deep. Shallow tanks less than 1m generally don't work very well unless the flow rate is slowed right down to suit but this tends to make them somewhat redundant, as brushes tend to work better.

Example: If a vortex tank were 0.9m diameter (90cm) the surface area would be 0.9 x 0.9 x 0.75 = 0.61m2 so the average flow required would be 0.61 x 15,000 = 9150 litres per hour. As with any system the flow rate will differ from one pond set up to another so some amount of trial and error will be needed to get the best results. Although slower flow rates generally work better.

Baffle tanks: An older system that works very well and requires very little maintenance but needs a large tank to work, as the flow rates are very slow. The baffles are set at angles from the vertical plain of 32.5° to 45° depending on the flow rate; this needs to be done very accurately. As the water enters the tank it works its way up to the surface, whichever way it flows it will travel the same distance and therefore the same speed, depositing the waste particles one the way. If you have the space this is a very good system but is difficult to set up and get started.

Pressurised sand filters: Sand filters were designed to remove low volumes of small particles in relatively clean environments, so the garden pond is not an ideal situation for them. They are expensive to purchase and run, as they require a large pump with a high pressure rating to clean them. They require regular cleaning as they clog very quickly and in some situations can cause gas bubble disease (not a disease but a condition similar to the buoyancy in effect).

The principle of the filter is to force the dirty water through a sand bed. As the water passes through the sand the particles are left behind. The more the filter clogs the better it works but the flow rate slows down as well. The multi port valve is a single valve that allows the control of the filter by easy selection from the settings. It allows normal filtration, back washing for cleaning or rinse to stop waste entering the pond and recirculation. This allows work to be carried out to the filter without stopping the pump, although it is always recommended to switch the pump off while working with water.

The back wash cycle requires a large pump, typically a 1 hp, to back wash a 60cm filter. There is 100kg of sand in a 60cm filter to lift during the back wash process, as the clean water is forced to the bottom of the filter and then up through the sand and out to waste. There is now a new filter media for sand filters that does not bio foul and can be used for ponds with little extra maintenance.

Pressurised Filters: A quick note on these filters. Please do not buy them unless you are absolutely sure you want one and you have sized it correctly (read the instructions, that's what they are for).

Let me explain: They treat nothing like the volume the packing would have you believe.

Example: Oase 6000 treats 4000 litres if there is no fish and only 1000 litres (220 gallons) if you have fish.

Their Downfall: As they clog the water flow slows down and therefore the pump rate slows and this causes the oxygen levels to fall, as the pump is not pumping sufficient water to oxygenate the pond then the fish die, simple. A normal filter lets water down the over flow back to the pond and would still oxygenate the water resulting in no dead fish.

The pumps used to run these are generally high volume low head pumps (Aquamax, Titan). As back pressure = head this means you need a large pump to run a small pressure filter. For Example: Oase recommend an Aquamax 16000 and a Filtoclear 11000 to run a 2500 litre (550 gallons) pond with fish. If you used a normal gravity exit filter (i.e. Above the water level) you could use an Aquamax 4000 and a Biotech 5. Not only would the Biotech do a better job but also it would save over £130 per year in running costs, a good saving in the long run and the set up cost would be about £200 cheaper as well.

Please Note: This is not a dig at Oase, they make good products. The problem is the people who miss-sell the equipment, as they don't read the instructions. Check before you install the product, as the manufacturer does not have to refund if it is installed outside of their guidelines.

Biological Filtration

The next stage after the removal of the solids is the reduction of dissolved waste such as ammonia and nitrite. The biological filter is an area devoted to the support and concentration of bacteria needed to break down the waste. The waste from the fish is the start of the nitrogen cycle in the pond; this is an important biological cycle that nature uses to recycle the waste that the fish produce.

The figure shown is a very simplified one and shows the basic actions in the cycle. The start is when the fish are fed, they covert the food into body mass and use some as energy to live; the waste is excreted in the form of ammonia. The naturally occurring bacteria in the pond, nitrosomonas, break ammonia down to nitrite using oxygen. Once this has been done the next set of bacteria, nitobacter, get to work converting nitrite to nitrate; this process also requires oxygen.

The final product, nitrate, is plant food, The pond plants, good and bad (algae and blanket weed), need this to grow and will consume as much as they can. The nitrate level will rise, as it is unlikely that the plants in the pond will be able to keep up with the nitrate production.

What Bacteria Want

The bacteria have a few simple requirements, the first being oxygen. Oxygen is needed to break down ammonia to nitrite and nitrite to nitrate. The food required is readily supplied by the fish in the form of ammonia. The important issues that effect biological filters are:

• Oxygen supply: Without oxygen the bacteria will die, anaerobic bacteria will take over and convert nitrate back to nitrite which will poison the fish. The oxygen levels need to be as high as possible. The addition of air stones to the filter will help the process.
• Temperature: The higher the temperature the better it will work until the oxygen level drops off at very high temperatures.
• Chemicals: Many treatments will affect the filters efficiency by either reducing the oxygen levels or by directly killing the bacteria.
• Water flow: As the water brings both the food and oxygen, this is a very important factor. A good flow rate to aim for is 8 to 10 times the volume of the bio-filter per hour. The filter will work at much higher speeds but this is the ideal rate.

The Filter Media

The filter media is the bacteria's home and should offer a large surface area, as the more bacteria you can support the more work the filter can achieve. The media needs to be rough so that the bacteria can grow on the surface. It must have a good structure so that water flows evenly through it so that no dead areas can become anaerobic (oxygen deficient).

There are a huge variations of medias on the market today ranging from lava rock to plastic sheeting. All need to fill the following basic needs of a good filter media.

• They should be non-toxic and contain no recycled material, as it is difficult to confirm the content of a recycled product.
• Media should have a large surface area to volume ratio. The more bacteria the filter can hold the smaller the filter can be.
• The media should contain no glue, as it will eventually fail and pollute the water.
• It must be easy to clean. Very important if the efficiency is to be high as dirty filters consume more oxygen and do less work.
• The media must not clog. If the media clogs it will turn anaerobic and convert nitrate back to nitrite.
• Ease of handling and installation. This is often a considerable problem with medias that have small components, as it can be difficult to hold them in place or support them.
• The medias should not compact or lock together making cleaning impossible. Most of the ‘aqua rocks’ sold compact down with time and are very difficult to clean effectively. The compacting media also creates tracking, where the water only runs through in areas of the least restriction this effectively stops all the media working and reduces the amount of work that can be carried out by the filter. Tracking also increases the likelyhood of the filter blocking and creating anaerobic areas.
• Weight is a key factor in filter media as it affects the tanks required for the filter. Gravel weights about 1500kg per m3 this will have to be taken in to account when building and installing the tank. This will also affect the cost, the media might be cheaper but if the tank is more expensive. The lightweight media would be a better bet and easier to clean.
• The media must have a known surface area to volume ratio as it makes it very difficult to work out the filter size if the media is random.

The first media available to pond keepers was gravel and now the choice has become somewhat difficult but there is a simple method to choosing the media for you.

Gravel: A cheap media but has most of the problems mentioned above; it compacts down, is difficult to clean and very heavy.

Aqua rock: This is either crushed lava rock or clinker, the waste from kilns. It is very difficult to clean and has a poor surface area to volume ratio.

Lytag: An expanded clay pellet that has a large porous surface area and often floats. It can be difficult to clean and clogs easily.

Flocor: A plastic media created for sewage farms; very good surface area to volume and relatively inexpensive. Flocor is used on commercial fish farms. The light weight makes filter construction easy and it is easy to handle. The only down side is that, as a loose media, it traps dirt and is quite difficult to clean.

Japanese filter mat: This is a jumble of plastic fibres bonded together. While recommended by fish retailers and koi experts it is a relatively poor media as it has a number of basic problems. It is glued together and the glue eventually fails leading to small plastic fibres floating around the filters and finding their way to the pump, which they then clog. It has a random construction and no set surface to volume ratio. The mat is a mix of different plastics and no data is available on the make up of it. The mat needs to be cut up and turned into cartridges as it clogs quickly otherwise. It is difficult to clean.

Plastic media: There are a number on the market but many have problems when it comes to the surface, as it is cheaper to produce a media with a smooth surface so most do. This leaves the bacteria with nothing to hang on to so this type of media will take a very long time to a mature. If the water flow is too strong the bacteria will get washed off.

Bio balls: One of the first effective plastic medias; can only be used in a filter where the first stages remove all the solids as it clogs quite quickly and is difficult to clean. Bio balls offer a very large surface area but the complex nature of the design is difficult to produce and this is reflected in the very high production cost. Best used in trickle towers.

Bio glass: A very expensive media, as it is a honeycomb of glass made into small sections in the shape of a doughnut. The surface area is incredibly large, about 100 times the area per m3 of most other medias. As with most good things it has never proven its self in ponds due to the high particle waste levels in the water, as the surface of the media is so fine it clogs almost on day one and is of little use after a few weeks. Bio glass is ideally suited for indoor tanks.

Bio-Block: Bio-Block a very good media that is not only easy to clean but has a large surface area to volume ratio. Its open structure is not prone to clogging and offers an even flow path for the water, therefore no tracking. It is strong and easy to install as it in comes in blocks that can be cut to fit the tank. Very popular with commercial farms due to its high area to cost ratio making it value for money. Can be used for settlement and biological sections of the filter.

Ready Made Filters

The simplest filters consists of a foam prefilter on the front of the pump, these work in small ponds up to 500 litres. They require regular maintenance and if left too long will starve the pump of water and cause the pump to fail. Not really suitable for ponds where lots of fish are housed due to the relatively poor filter capacity and high maintenance.

The next step up in filters consist of a plastic water tank with a spray bar spreading the water across thin sheets of foam of varying grades this collects the particles and any dirt in the water, for the biological action to take place a plastic ring material, which offers a large surface area to a volume ratio, is used for bacteria to live on.

The next stage is a multi-chamber filter. In this type of filter the different filtration stages are split up allowing the use of more media and when cleaning the biological media is left undisturbed.

The filter shown is a simple 3 chamber model that uses brushes in the first chamber for settlement and then has two biological chambers for the bacteria. This offers the chance of using two different medias which gives a more diverse habitat for the bacteria to grow in which will improve the filter's efficiency. There is a perforated tray to hold up the media and allow the water to flow underneath to the next chamber. The settlement chambers have drains to waste for easy maintenance.

The multiple vortex filters are a newer design popular with manufactures as they are cheaper to produce and stronger than ‘box’ shaped filters. The come in 3, 4 or 5 chamber styles and the media choice is up to you. The vortexes are normally very small and the flow rates recommended are far to quick. To get the best from these you should buy larger than suggested by the manufacturer and slow the flow rate down. All chambers have drains making cleaning easy and quick. They are best used as gravity fed filters otherwise the pump breaks up the solids making the collection more difficult.

Fluidised filter beds: These are biological only and require a good prefilter to stop solids from entering the filter system. They are very effective and very compact for the capacity they have. They have a set of requirements that must be provided for them to work.

• High capacity pump to start and lift the media into suspension.
• The flow rate must be constant any large variation will stop the filter from functioning. Too much and the media will end up in the pond, too little and the media settles out and will turn anaerobic.
• Well-oxygenated water to function as a vast amount of bacteria is held in a very small volume.
• The filter must be vertical to work correctly, otherwise the media will settle out on one side creating an anaerobic section.

Fluidised filter beds must never be used without a normal biological filter. They are by nature unreliable and should be considered an additional filter rather than the main one. They are affected by chemicals and die quickly if the water flow stops because the media stops moving and sinks to the bottom, compacts down and the bacteria run out of oxygen.

Fluidised filters are normally the domain of marine systems and commercial indoor recirculation systems but they are always used as an additional filter because of their difficult nature. The materials used to build them should be stainless steel or a tough industrial plastic. The nature of the filter with moving sand particles makes them very hard on the filter container and they will soon wear out thin plastic or fibreglass tanks.

Trickle towers: In the commercial world these are the most popular filters as they offer the best of all filtration methods (again they offer only biological filtration). Because the media is suspended in air the oxygen content is about 20 times higher than in water so can support 20 times the bacteria of a traditional submerged filter. The action of water falling down the media exposes it the to air, helping to remove ammonia which is lost to the atmosphere as a gas and this reduces the work the filter has to do. At the same time it reduces nitrate, which in turn reduces algae growth as there is less food for it.

The design is simple and reliable; it has few requirements and because of this is easy to set up and run. The tower can be lightweight as it holds no water and only has to hold the filter media upright and contain the water as it falls through the media. It is even possible to use thin plastic sheeting as a tower if the media will self-support. This reduces the initial outlay.

The spray bar needs to spread the water evenly over the surface of the media. This can be achieved best with a rotating spray bar as it is constantly moving the water across the surface and, as it is not a continuous stream, it does not wash the bacteria off the media. Another method is a spreader plate, a tray with holes drilled at set distances all over the base, which allows the water to spread over the media. The filter's height needs to be around 1.2m minimum to have any real effect.

The filter's efficiency can be improved as the fish increase in size by simply adding an air pump to the filter base so that air is allowed to work its way up the filter and out of the top, this increases the oxygen levels and removes any ammonia that is forced out of the water. Another easy way to increase the airflow is to add an extractor fan to the tower to pull air through the media.

The two drawbacks of trickle towers are:

• Height: they need to be tall to work and they have to be above the water level of the pond.
• Temperature: Because they pass water through the air they will exchange energy between them. In warm weather they will heat the water and in cold weather they will cool the water.

If power is lost they can survive undamaged for days without much loss of efficiency, as the air in the tower is humid and full of oxygen.

Starting the biological filter: All biological filters take time to start processing waste, as the bacteria need to build up to sufficient numbers to start working. Adding a bacterial filter-start such as ABA can reduce the build up time. These contain all the nitrifying bacteria in a solution that suspends them in an inactive mode. When they are added to water the bacteria start to function and multiply. At this point all ultraviolet filters should be left off to avoid any sterilising effect they may have.

Order BacterLife P online from Orchard Fisheries

The pond goes through ‘new pond syndrome’ where the addition of excess fish leads to a wipe out or loss of most of the fish. This is caused by the filter not being able to break down the fish waste because it is immature and the fish die because they are swimming in their own waste. When starting a new pond it should not be stocked heavily for at least four to six weeks to avoid this effect and fish should be added in small numbers after being quarantined first. Testing the water in your new pond will help you to determine when it is safe to add new fish as the start up of the filter follows a simple path.

• As the first fish are added the waste is produced as ammonia. This shows on a test kit and the level rises over time until the bacteria appear in sufficient numbers to convert it to nitrite.
• The nitrite will start to rise as the ammonia is converted and will continue until the bacteria that convert nitrite to nitrate reach the correct numbers and the level then drops.
• The nitrate level rises until it is either water changed out or used by plants.

The speed at which the filter starts is affected by the water temperature and the amount of oxygen available. The higher the oxygen content the faster the filter will establish itself.

Note: Every time you add fish to the pond the filter has to build up more bacteria to help with the increased load. Often this is called shock loading and many filters, especially small filters that are on the limit of their capacity, fail and a complete breakdown leads to instantaneous waste build up and fish deaths. So when adding a number of fish to your pond always check the levels for a few days to make sure this does not occur.

Sizing the filter: The creation of a biological filter is very simple and almost any material can be used as a media, from gravel to Bio-Block there is no restriction on size or type. As long as you follow the requirements of the bacteria it will work.

Sizing can be difficult as people's ideas range from 1/3rd the size of the pond to a tiny 25 litre tank. A rough guide is to allow 1 litre of filter media for 1 gram of food per day. A fish requires 2% of its body weight per day to live a healthy life with normal growth.

Example: A pond holding 10 fish at 60cm in length (a fish this size would weigh about 3kg) would need 3000 x 2% = 60gms x 10 = 600gms of food per day so a biological filter holding 600 litres of media should be fine.

It is always best to over size your filter as this formula only works well when the temperature is constant (which it never is) so allow 10-15% extra media to cover any variations or extra fish that find their way in to your pond! When building a filter allow some extra space around it for extensions should you need it.

Nitrate Filters

The last process of the aerobic filter is the production of nitrate. Although it is not toxic until levels are very high, in excess of 150mg per litre, the lower the levels the better it is for the fish. In their natural habitat it would be less than 1mg per litre. There are three ways to remove nitrate effectively; with a filter, with plants and the last with exchange resin.

Denitrifying filters: A simple denitrifying filter is a one that supports bacteria that require oxygen free conditions, anaerobic. Denitrification requires a source of carbon to work, the normal approach to this is to add either methanol or sugar as the supply, the bacteria then break down the carbon source and nitrate to produce carbon dioxide, nitrogen gas and water.

The filter tank needs to be set up after the normal biological filter to take advantage of the low oxygen and high nitrate content of the water exiting from here. Ideal conditions for denitrifying bacteria are oxygen levels of 2-3mg per litre and nitrate of 30mg per litre or more. If levels fall below this hydrogen sulphide will be produced which not only smells like rotten eggs but is also toxic to the fish.

The water needs to spend 2-4 hours passing through the denitrifying filter for it to work. For example if the filter holds 100ltrs the flow rate should be between 200 to 400 ltrs per hour. The outlet should return the water to the beginning if the bio filter to reduce any nitrite that may be in the return water. This is a difficult system to set up and has a number of problems that can cause major disasters if they go wrong. It is not for the busy person, as it requires time to be spent checking it each day and back washing it weekly to remove the sludge build up that occurs. But it uses nature to do the work and is in general a very good method of nitrate removal.

Plants: The most natural way to remove nitrate from the pond water is to use plants. As plants grow they use nitrate to build the leaves and stems that they use to collect sunlight to use to produce food for growth as they grow they require more nitrogen.

The best way to grow plants for vegetable filter is to grow them in a shallow water coarse that is fed from the biological filter, where nitrate production occurs. The more the plants are cut and removed the more they grow which means they will remove more nitrate. The plants also pick up other toxic metals and chemicals so they are good for purifying the water as well.

The simpler the filter design the better it works and the maintenance is also simple. The vegetable filter should be large enough for the flow from the bio filter and long so that the water takes a reasonable time to travel through the filter giving the plants a chance to pick up the nitrate as it goes past in the water. The filter must be free flowing, as you don't want it getting clogged and overflowing. The plants will need regular trimming to ensure the maximum growth is achieved. The only down side is the plants do not grow well in the winter months when the weather is cold.

Ion Exchange Resin: The last method is to use resins that remove nitrate in exchange for another ion, usually sodium chloride (common salt). The resin is ‘cleaned’ or recharged by washing in a salt solution of 100gms per litre water, each litre of resin needs 5 litres of salt solution to recharge it. This is the easiest method for removal if the space for vegetable filter is not available; it is compact and very good. The levels can be reduced to less than 25mg per litre which is often lower than the tap water used to top the pond up. It is difficult to say the amount of resin required for each pond but a starting point is 1 litre of resin for each 5000 litres of pond volume. You can always increase the amount of resin if need be or if it is too much there will be less cleaning to do.

Ultraviolet Systems

Ultra Violet Clarifiers are probably the least understood and the most poorly sold pond products on the market. They are sold to cure every problem, from blanket weed to parasite infestations, when in fact the standard pond unit only cures one pond condition, green water. The idea is to pass the green water through a bath of UV light which effectively stops the algae's ability to reproduce. The water then clears and all is well for as long as the bulb lasts. Then the pond goes green again if the pond plants have not grown enough to compete with the algae.

All UV systems work by circulating pond water around the UV bulb, whether it is the older double-ended bulbs or newer single ended type, which is encased within a quartz sleeve to protect the bulb from contact with the water. The outer case directs the water along the outside of the sleeve, ideally in a spiral pattern allowing for the maximum contact time with the UV light. No water can escape the light therefore giving a better kill rate and higher efficiency. There is only one manufacturer producing such a unit with the spiral flow built in. UV light is absorbed very quickly and there are a surprising number of factors that affect the UV’s efficiency.

• The speed at which the water flows through the unit; too fast and the contact time will be too short, too slow and the kill rate will be higher but the algae will grow faster than it can be killed.
• Suspended particles in the water; the more debris the water is carrying the more light will be absorbed and the less work the UVC will do.
• Dirt on the quartz sleeve; if the sleeve is dirty the light cannot get out to work.
• Chemicals in the water; any dyes that are used as treatments will affect the light's ability to travel through the water.
• Poorly designed UV units; this is the biggest cause of UVC not working. There are a number of designs that are very inefficient and should be avoided. Any unit that suspends the bulb above the water is asking for trouble, water can splash on hot the bulb and electrics. Units that pump the water through a quartz sleeve are very poor as the amount of UV light that gets from the bulb to the water is so small, about 10% or less. A properly designed sleeved UV bulb will put about 99% of its output into the water, therefore giving the most efficient and safest use of UV output.

There are two types of Ultraviolet systems available; the standard pond Ultraviolet Clarifier (UVC) and the Ultraviolet Steriliser (UVS). The design of the UVC is very similar but has a number of factors that separate it from the UV steriliser. A large design difference between them is the gap between the quartz and the outer casing. The UVC is only designed to kill algae so the gap is large, up to 4cm, and allows a high throughput and low resistance to small pumps. The output near the casing is very low as the water absorbs the UV light. This means the casing material can be thinner than on a UVS. The gap on a UVS can be as small as 0.5cm and in most cases it is about 1-1.5cm. This means the UV level is very much higher and at this dose even bacteria, viruses and parasites can be killed. This dose also takes its toll on the casing as the UV levels are very high, therefore UVS are normally made from industrial plastic materials or stainless steel. The flow rate has to drop to very low volumes, as the contact time required to kill bacteria etc is far longer than to kill algae and this means to service a pond many more UVS would be needed against a UVC just to control algae. This explains the higher cost of sterilisers but they do a totally different job to clarifiers.

UVSs are best put after filters, in the return if possible, as they work better if the water is free from floating particles. UVCs will work on either the dirty or clean side of the filter as the UV dose is less important. One of the most important factors affecting the UV units are the bulbs; they have a relatively short life, between 8000 to 9000 hours continuous use. Aquatic retailers often tell pond keepers that the more UVCs on a pond the better, this is untrue. Once the water is clear it is pointless to have more and more UVCs as they are unable to kill bacteria and parasites as the UV dose is too low and the contact time too short.

There are a few myths about UV systems in general:

• 'They sterilise the pond and increase the fishes immunity to disease.'
  This is not true as it would be impossible to create a sterile environment in the open area of a garden and UVCs do not sterilise at all. UVS only keep water-borne diseases under control, not anything on the fish themselves.
• ‘UVCs control blanket weed.’
  This is unlikely, as the blanket weed does not pass through the UV chamber.
• ‘UVs affect the water chemistry and this can kill the fish.’
  No tests have been done that prove this and I have never seen anything to even suggest this could happen. One of the main reasons UV units are used to sterilise drinking water is that they do not affect the water chemistry.
• ‘If the bulb is glowing it is still working.’
  This is also untrue; the reason the manufacturers of the bulbs give you a life for optimum use is because they know it lasts this long and, as the UV light is invisible, the fact that it glows and you can see it is not the point.

There is no doubt that Ultraviolet filters play an important part in filtering the pond but, if the biological filters are the correct size and the pond is not over stocked, there is no reason that the UV cannot be used for the start of the season and then turned off as the filters start to work.

Summary

There are always differences of opinion when it comes to filtration and there will always be the pond that survives with nothing and is ‘perfect’ but unfortunately these are few and far between. Nothing looks worse than a green pond and with modern pumps and filters there is no excuse for it. If selecting a ready made unit always read the instructions as they will always say the filter will treat a huge pond but, if the factors they mention are taken in to account, the filter's capacity diminishes very quickly and a 10000 litre filter soon only filters a 2500 litre pond. Once you have used it, taking it back will be difficult.

You cannot over filter a pond, as the larger surface area means that it will take longer to clog and can cope with longer periods between cleaning. The filter will not be affected by a sudden increase in fish population, as the excess filter media will just kick in without a struggle. Excess filter capacity is a good back up and if the budget will stretch to it is a worthwhile investment.