Many people confuse a prefilter with a biofilter. It’s easy to do. Those who manufacture prefilters often call them biofilters on the packaging. They shouldn’t be able to get away with it but they do. Prefilters are used in conjunction with submersible pumps to strain large debris so that it won’t get caught up in the pump. They’re often built onto certain pumps or come as optional attachments. We like to construct our own with pump baskets.
For submersible pumps a prefilter is not an option. It should be considered a "must". Leaves and muck can clog the intake area of the pump. Debris and even fish can be sucked up into the pump and cause damage to the pump and, of course, will kill the unlucky fish that happens to get caught in the suction. A prefilter protects your investment in the pump and, in a few cases, the fish.
Quite often someone calls who has a submersible pump and wants to know why their pump has stopped working or why the flow of water is no longer as strong as it was when the system was installed. Nine times out of ten it is that there is no prefilter and the pump is clogged or the prefilter, itself, is in need of a cleaning.
Submersible pumps, whether protected by a prefilter or not, will need cleaning — and during the peak season once or twice a week. This involves reaching down into the water and pulling up the filter along with the filter basket or attached prefilter. Strong blasts from the garden hose usually works fine to clean the prefilter. Without the prefilter you almost have to take apart the pump and remove the debris each time. Now you see why we prefer a bottom drain and outside pump operation — much easier on the back!
If a bottom drain is out of the question you will have to contend with the problems associated with a submersible pump. Cleaning and prefiltering will be the key to keeping the system up and running for years to come.
The most effective prefilter you can place on your submersible pump is one that is coarse enough to allow small dirt and sand particles to travel through the pump and get trapped in the outside biofilter where it is easier to clean. At the same time the material of the prefilter must not allow leaves and larger objects (especially fins of fish) to attach to the water intake of the pump.