What to do when a biological filter starts leaking and your pond losses water

If you have a substantial water loss and you have an external biological filter it may be best to check this out first in the process of elimination, since you want to preserve its effectiveness for biological filtration. Turning the thing off for a long period whilst you check everything else is undesirable.

CLUES:- If there has been very little maintenance to your biological filter of late and we are in the throws of a long hot summer then this could be the culprit for substantial water loss. As the filter box is continually being fed with ever more grimy water, and because the cycle of life in the pool increases in speed as the water warms up, then there comes a point when the bacteria are unable to cope with breaking down of all this organic matter. The detritus begins to build up. Eventually whatever medium is being used for filtration clogs up to the extent that the clogged up filterbox overflows in one of its chambers. Look for drips out from under the lid. (See also Filter Problems)

[P.S. Bear this in mind before you go on a long summer holiday]

If you feel that this is something that might happen at any moment whilst you are away, see 'N.B. ENSURED WATER LEVEL'

If there seems to be drips from under the edge of the lid and you have just cleaned out the filter and the pump, then the fresh force and vigour of the newly cleaned system is sending the spray every way and which way under the filter lid. Therefore either turn down the pump's supply of water with a gate valve or somehow baffle the spray so that it gets knocked down into the filter without rebounding everywhere.

Filters have other inherent problems. They are generally of fairly rudimentary design and in the past in particular, were put together out of materials and products that were designed for other purposes. So you have a mixture of incompatible materials held together by other materials that behave differently as temperatures fall and rise. In the cold, hose-tails become brittle and crack; in the heat, jubilee clips expand and hose becomes soft. What was a tight fit between ABS plastic, HDP and PVC now lets water flood through.

An Ultra Violet Clarifier and biological filter working together but all those fixings and fittings not only restrict water flow, they are weak points at which leaks can occur.

CHECK all the fittings, especially at the inlets and outlets.

Water from the main outlet can defy all the normal laws of gravity. Water that should be being efficiently deposited from a wide 40mm tube back into the pool will drip back up the tube towards the filter on the outside, even if it seems to be sloping down from the filter. It will then dribble down the side of the filter and away from the pool.

CURES:- Last point from above first.

An elastic band or a tight "O" ring around the end of the outlet pipe deters any drip back.

With fittings, flanges and threads. Hose-tail threads - where appropriate use plenty of plumbers PTFE tape.

Where hoses push onto hose tails and on jubilee threads, use lashings of Vaseline or waterproof grease. It will help you get them undone for maintenance too.

Flanges can be sealed with grease but the rubber washers that have come with the fitting, that are now lost, can be copied using Butyl or Rubber liner samples from your local Water Garden Centre. Alternatively, cut up an old hot water bottle.