Fish Ponds

There are many sorts of filters sold for fishponds and one or two more types that you can make yourself. There are some that are small pre-filters which are just foam blocks and fit onto the inlet of the pump. A second sort is a small inline filter which is a plastic container sealed by a rubber ring containing a foam block. Some small filters are built into a waterfall that hides them completely. However with all of the small filters they are too small to do any real good and can only help the problem of green water a little bit in all but the smallest of pools. One sort of small filter that does claim wonderful things contains sintered glass. In my opinion, they are overrated and will only really work if the pond is already fairly clean.

One filter which can work efficiently if set up right and is often used in Japan, in large Koi ponds, is an under gravel filter. This means having a deep layer of gravel in the bottom of your pond and a network of pipes embedded in it with holes drilled in connected to your pump. The water is then drawn through the layer of gravel which filters it.

The drawback with this system is that it is a major job to clean it out when it becomes overloaded with dirt. The most widely used successful filter is a filter box which can cost from about £40 treating 500 gallons, to very big ones treating many 1000s of gallons and costing hundreds of pound.

Most modern box filters sold commercially contain reticulated foam, or filter brushes and some sort of plastic medium. There are usually 2 or 3 layers of foam that is graded from course to fine. The foam is a special sort that does not contain cyanide compounds that are extractible in water and highly poisonous. The foam is shaped like an egg box, or reticulated to give it a larger surface area. Filter brushes are a bit like a chimney sweeps brush and like the foam will filter out the larger particles that would otherwise pass through the filter. The plastic in the bottom comes in many shapes from different manufacturers, but all of it does the same job. The water passes through it easily so it does not get blocked, but it has a large surface area on which the nitrifying bacteria can live.

You can of course make your own filter using a coldwater header tank and buy the filter medium to go in it from your local aquatic retailer. Foam filter pads, filter brushes and various forms of plastic media are readily available as are other sorts of media.

Early, homemade box filters used to be simply filled with gravel instead of modern foams etc. They worked on the same basic principals that traditional sewerage stations filters operated and worked fine in their own way. They were cheap to set up and the filter medium (gravel) was cheap to replace, but the main advantage of modern, lightweight, filter mediums over the early gravel ones, is that they are very easy to clean.

Besides gravel, Canterbury spa chippings and Lytag are often sold for old-fashioned filters, but Zeolite is sometimes available and much better. Zeolite is a naturally occurring mineral that has the unusual property of absorbing chemical impurities in water, so it will remove the nitrites and ammonia from a fishpond. Then, when the Zeolite ceases to be effective, which you can tell from regular water checks, you can soak it in a strong salt solution to clean it and make it effective again. The drawback is that it is quite expensive to buy.

Another type of filter sometimes used is a sealed, pressurized filter. The main advantage of this type is that it can be external to the pond for ease of maintenance and this also allows it to be hidden away from the pond.

In the past sealed, inline filters were either too small to be effective on a decent sized pond, such as the early foam filled ones, or else the better ones were prohibitively expensive, both to buy and to run.

Now several leading manufacturers have come up with a new design which is both more effective and which sells for a reasonable price. These new designs incorporate modern filter media instead of sand unlike the older more expensive models. The outer cases of the filters are of course made from durable plastic and should give many years service.

The main problem with this type of filter is that it can only hold a certain amount of muck before it blocks up and needs to be cleaned out thoroughly. Backwashing on a regular basis may be necessary to keep it in operation. Another point to bear in mind is that most modern pumps are very low pressure, but high volume and the very nature of the filter offers some resistance to the flow of water through it. The resistance will increase dramatically as the filter absorbs the particles of suspended matter out of the pond water, so to work the filter effectively you will need a more powerful pump. This is especially true if the outflow is to be used to operate a waterfall as well as this will be reduced.

Alternatively an older style pump that has a less efficient flow rate, but a higher operating pressure may be a good choice. It is a simple matter to decide whether the pump is low pressure, or not by measuring the outlet on the pump. The narrower the outlet the higher the pressure assuming that the flow rate is the same.

In other words most pumps use a 1 inch or 1 ¼ inch outlet nowadays, but if you can find a pump with a ¾ or even a ½ inch outlet producing the same flow of water it follows that it must be higher pressure. This type of pump will use slightly more electricity, but this will be negligible compared to the cost of setting up the pond and filter system. On the other hand if the pressure in the flow of water is too high the filter won’t work at optimum efficiency anyway.

Another benefit that using a sealed filter has over the standard box type is that the outflow is still under pressure so this can be used to operate a fountain ornament.

One way to chemically remove impurities in the fishpond is to add an activated carbon filter pad to your filter. This is a filter-mat that has been impregnated with specially treated carbon that will react with any chemical in the water. It does have a limited life, but when the carbon is used up you still have the filter mat.

Settlement tanks are occasionally sold which aid filtration (see section called Improving Your Filter)

Ultra Violet units are also now available which will clear the fishpond and aid filtration (see section on Ultra Violet Systems)

When buying or making a filter you have to decide on the size and to do this you need to know how many gallons your fishpond holds. First of all work but the volume in cubic feet by multiplying the length, width and depth together and then multiply by 6.25. This will convert cubic feet into gallons. For the filter to work it needs to have the right amount of water going through it for the size of pond. The whole volume of the pool should pass through the filter every 2-4 hours. So a 1000-gallon pond should have about 250-500 gallons going through the filter every hour.

When choosing a pump to operate a filter, always allow a little bit extra on the quoted flow rate so that when the flow rate drops, as the pump gets dirty, it is still doing its job properly. To save having 2 pumps to operate a filter and a waterfall simply site the filter at the top of the waterfall so that the water runs out of the filter box and into the waterfall, but make sure your pump can cope with the extra height to the top of the filter.

When started the filter must be run 24 hours a day from the beginning of March till the end of October. This is not as expensive as it sounds because most pond pumps are very efficient and use only a little more electricity than a light bulb. Typically a pump will cost only 2 or £3 a week to run which is not very much expense for a hobby.

When the filter has been turned off for the winter it should be thoroughly cleaned out in the spring, if it has been left full of water, before it is turned on again because the aerobic nitrifying bacteria will have died off and been replaced by anaerobic bacteria which are poisonous to fish.

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