Filter Related Questions

How fast should the water in Koi ponds be circulating through the filter system?

On ponds up to about 12000 litres, all the water in the pond should pass through the filter in a 1 to 2 hour period for best results. In larger ponds a turnover rate of every 2 - 3 hours has been found to work quite adequately. Remember to choose a pump that will move the correct amount of water per hour at the height and distance you need to pump it. Most pumps quote the maximum capacity they will circulate and this will drop dramatically if you need to lift the water up to a high waterfall for example.

What is the best type of filter system, gravity fed or pump fed?

Although there are many thousands of successful ponds that use a pump fed system, gravity fed filters have several advantages. In a gravity fed system the top of the filter is installed at the same level as the top of the pool, the pool is connected to the filter by 3" or 4" diameter rigid pipe work via a bottom drain. The pump is installed at the far end of the filter and returns water from the filter to the pond. Because the filter and pond are connected and at the same height, more water flows from the pool by gravity to replace that being circulated by the pump. This constant flow of water draws debris from the pool base into the filter where it is easily settled and removed. In a pump fed system any waste food or feaces from the fish are drawn through the pump, chopping them into fine sediment which is much harder to settle out in the first chamber of the filter. The pump is also at risk of clogging, whereas in gravity fed system the pump is circulating water that has travelled through the filter and is very unlikely to clog.


How does a Koi pond biological filter work?

The fish in a pond excrete large amounts of ammonia as a by-product of their metabolism, they are in effect swimming around in their own toilet. The main function of the filter is to act as a sewage treatment plant for your fish, it does this by providing a large surface area for bacteria to colonise. These bacteria break down the Ammonia and convert it first into Nitrite and then into Nitrate, which is relatively harmless to pond fish. The filter is in effect a living organism in its own right, hence the term biological filter. It takes five to six weeks for the necessary bacteria to become established in a new filter, during this period it is important to stock the pond gradually, adding a few fish every week. Pond filters must be operated 24 hours a day to supply the bacteria with a constant supply of oxygen and food. When the filter media needs cleaning it is important to wash it with water taken from the pond, cleaning the filter with tap water containing chlorine will kill much of the bacterial colony.


I have an ultraviolet light fitted to my pool, and the water is crystal clear, why do I need a biological filter?

The purpose of a pond Ultraviolet light unit is to kill off green algae and provide clear water to enhance the appearance of the pond, it has no effect on improving the water quality for the fish. With a U.V fitted it is perfectly possible to have wonderfully clear water that is full of dissolved metabolic wastes such as Ammonia and Nitrite that are poisonous to the fish. An ultraviolet light should be used in conjunction with a biological filter to provide water that is clean to look at, but also healthy for the fish.

I have fitted an ultraviolet light to my pond and the green water has gone but I still have blanket weed, why hasn't the ultraviolet killed this as well?

Ultraviolet lights only affect organisms that pass through their housing, they can clear up green water because they kill off the algae cells as they pass past the ultraviolet. Blanket weed remains anchored to the poolside and so is not exposed to the effects of the U.V unit.