Filtration Unit

Pond Tips -Pond Filtration

March’s pond tips concern the complicated subject of pond filtration. The first thing to say is that about 90% of the pools and water features we build have no filtration on them at all. The best way to keep water clear and oxygenated is to plant it heavily with submerged plants and not to overstock with fish. The plants absorb any free nutrients and light and leave none for the algae that make water green or grow into unsightly mats. There is also no need to move water with pumps as long as there isn’t a lot of organic matter in the pool causing an oxygen deficit. Indeed the worst cases of filamentous algae I have seen have often been where a big pump is running a cascade or waterfall. An ordinary garden pond with goldfish has absolutely no need of a pump or any filtration if it is planted up properly. Filtration is only necessary if you have more fish than the pool can cope with. A standard box filter will help by trapping suspended solids and the Ultra violet clarifier mounted on the lid will break up phytoplankton and keep the water clear. The problem with this is the nutrients are still dissolved in the water and sometimes you swap green water for clear water smothered in filamentous algae. All in all informal garden ponds are better off planted than filtered. You may find your local garden centre advises differently, but then they are selling filtration systems so have a vested interest in persuading you that you need one.

One case that does need “filtration” is a shallow cascade or rill, or formal pool or fountain, where plants will just get in the way of a contemporary or formal design. Here you may well need to go with filtration that traps suspended particles, a UV that kills phytoplankton and a dosing system that wipes out filamentous algae. This is the only way to guarantee clear water free from algae. Of course this adds to the cost and has to be maintained in the future or it will stop working efficiently. Many of the formal fountains that you see in public places have these systems hidden away below ground in plant rooms that cost tens of thousands of pounds. Not really what you want in your back garden.

Photo: A good filtration unit for a fish pond with a medium stocking density of fish – a vortex filter for solids, a bead filter mainly for biological filtration and a UV.