I’ve been perfecting this design for 2 years. It works well on my 300 or so gallon fish pond containing 12 assorted fish. The filter media is based on a 2’ x 4’ sheet of 2” thick rock wool (also known as mineral wool) available from mcmaster.com, the housing is a milk crate, the top is a piece of plexiglass. Before we go further it’s possible that this mineral wool contains a small amount of formaldehyde as a binder. But I’ve gone through 4 media changes in the last 2 years and my fish seem happy.
Cut the rock wool to snugly line the milk crate inserting the bottom piece last. It’s important that the rock wool extends about an inch from the top of the crate to make a good seal for the top.
Line the inside with a piece of filter fabric ( I used a 4’ x 4’ piece) then trim the fabric to allow a 4 to 6” overhang. Then tuck the overhanging fabric between the rock wool and the crate.
Line the inside of the filter with wire mesh, to keep the cloth away from the pump. Again keep the wire mesh about an inch below the top.
Make the top, you need a piece of Plexiglas or other suitable plastic to cover the top, use a hole saw to make a hole for the discharge line to pass through somewhere close to the center . Rinse and soak the filter with a garden hose, insert your pump and sink it in your pond, wrap a piece of filter fabric around the discharge line and secure it with a nylon strap push it down to seal the hole in the Plexiglas. You’ll need to weight it down with a stone to keep the top on and stop the filter from trying to float away.