There is a wide choice of filters available, catering for everyone, from the beginner with a small pond, to the specialist with a large pool full of valuable fish.
The main problem with smaller pump-fed filters is that they are not adequately equipped to deal with the solids loading – they fall down mechanically, requiring such frequent maintenance that pond-keeping ceases to be a pleasure. If you can afford it, and have room for it, opt for a filter that is larger than the minimum recommended for your size of pond. The larger the filter, the longer the intervals will be between cleanings.
A major drawback with the larger filters, but, is that they are usually highly visible. Downllow models have to be installed fully above the ground, and upflow versions can be only partially buried. If they are installed at the top of a waterfall it is quite simple to hide them, but then the greater ‘head’ (the distance between pond surface and filter outflow point) will reduce pumping capacity.
A pump-led, pressurized filter with integral UV clarifier is the answer. This can be sited anywhere around the pond – topping a cascade, buried in the adjoining ground to lid level or simply stood on a level base. Because water exits under pressure it can be piped up to a waterfall header pool, or simply returned via a hose to the pond.
Pressurized filters really force dirty water through the foam media rather than relying on
gravity, and so many more solids can be trapped before the foam needs a clean. Before this happens, signalled by a drop in exit flow rate, the foam will become a progressively more efficient mechanical filter. The UV lamp is contained within the pressurized canister and all water has to pass by it on its way back to the pond.
Filter pumps
The pump that drives a filter must be of the solids-handling type, quite different from the one used to power a fountain. Fountain pumps do incorporate a strainer, but this is partly so their impellers are not clogged and partly to prevent dirt particles travelling up to the fountain nozzle and blocking the jet.
Solids-handling pumps are designed to take all the debris out of the pond and into the filter. Their impellers, depending on the model, will handle solids from 8-10 mm in diameter, which means that the poor gardener is not constantly hauling up the pump to unblock it.

