Use a farm-style industrial plastic stock tank or any heavy-duty container with rounded sides. The rounded sides promote water flow, which helps to aerate and circulate the filtering pond water.
Seventy-gallon stock tanks filter 2000-gallon ponds. Fifty-five-gallon containers can handle 1500-gallon ponds.
Drill a hole near the top of the tank for the closet flange. Trace around the flange to size the hole and then fit the flange in the side of the tank, using the attachment holes to screw it in.
Secure the screws with nuts.
Seal the edges of the flange to the container with silicone. Cut a small piece of egg crate grate to fit in the flange opening to keep any filter material from floating through the opening and into the pond.
Create a “rotor” from the PVC pipe to sit on the floor of the tank. Fit the “T” connector to a length of pipe that sticks up in the middle of the tank.
Face the two elbows on either end of the crossbar of the T in different directions. This will send water swirling around unimpeded, in one direction, as it enters the tank.
The top of the T gets another PVC elbow to hold a threaded nipple that will connect to the pond’s pump.
Cut the egg crate grate to fit inside the tank and sit on one of the low ridges in the tank. Cut a hole in the middle for the T pipe to stick through.
Place the rotor in the bottom of the tank and set the grate over it, down in the tank.
Cut the filter pads so you can layer them all over the grate inside the tank. Large commercial floor scrubber pads last for years and provide plenty of surface for healthy bacteria to attach to so they can gobble impurities out of the water.
You can use other rocks or filtering material, but the scrubber fiber is effective and easy to clean and ultimately replace.
Attach the hose or pipe from the pond pump to the top elbow in the T that you have fitted with the threaded nipple. Disguise the bio-filter with rock, slabs of slate or foliage if the filter stands next to the pond.
The cleaned water will pour directly back into the pond. You can also run a hose or trough to carry the clean water from the flange opening to the pond.
Pour some beneficial bacteria over the filter material, turn on the pump and enjoy a clear, balanced pond.