Koi ponds usually have submersible pumps that circulate water through some sort of system that filters out materials harmful to koi and other pond fish. Pond filters, depending on the size, can be very expensive. An alternative is a homemade pond filter that works as well as some of the more expensive filters on the market.
Some things you will need to make a homemade pond filter are a plastic garbage can or other plastic container with a lid, a garden hose, a valve, plastic scouring pads and a piece of foam. You will also need some silicone sealant and a craft knife.
Size of the filter
The size of the filter you need to make depends on the size of the pond. If the pond is 2,250 litres (500 gallons) or more, use a full-sized garbage can. With smaller ponds, use a smaller plastic container with a lid.
Preparing the container
Use the craft knife to cut a hole toward the bottom of the container on the side. Make the hole just large enough for the valve. The valve can be plastic or metal, but it needs to be one that a garden hose can be screwed into. When you have the valve in place, seal it with silicone. You may have to let it sit overnight to make sure it is dry. Cut a hole in the top of the lid just large enough to fit the hose of the submersible pump.
Place the plastic scouring pads on the bottom of the container and use enough to go half way up the sides. Cut the foam to fit the shape of the container and place it on top of the scouring pads. There are a number of other filter materials to use, such as lava rocks and river rocks, but scouring pads make the best bio filter. Good bacteria grows in the pads and eliminates bad bacteria that is harmful to fish.
If algae becomes a problem, place two pieces of foam on top of the scouring pads and put barley straw between the two pieces. Give the straw at least two to three weeks to kill the algae in the pond.
Hooking up the filter
Place the filter above the pond in a place where the hose from the submersible pump will reach to the top of the lid and push the hose through the hole far enough to reach the foam material. Connect the garden hose to the container and put the opposite end in the pond. The garden hose should reach the bottom of the pond, but not lie on the bottom. You may have to cut it off if it is too long.