The drawing at the top of this page is a quarantine tank I put together for Dale and Patti Gingerich. It can be run with just the barrel filter when the ball valve is fully open or it can be run completely through the 4 barrel tower with the ball valve closed or a combination of both by partially closing the ball valve. This filtration system would handle a 1000 gallon pond without a hiccup.
Our commercial site: http://www.koivillage.com
Use a 1-1/2" PVC cross. You can get them in white schedule 40 PVC at Lowe's, Home Depot or other plumbing sources. Use a hole saw and cut a hole in the center to accept a 1-1/2" PVC pipe.
You want outlets of the PVC cross to fit tightly in the holes. I beveled the ends of the outlets to get them started in the holes and forced them the rest of the way.
Once I assembled the PVC cross and the four sections of ABS pipe I glued a short piece of 3" ABS pipe in the center (not shown) to make them stable. Always use "All Purpose" glue when gluing ABS.
Drill 3/8 or 1/2" holes in the end of four 4" ABS end caps and glue them to the bottom ends of the pipes. Fill the pipes with bio balls or another plastic media that has a good flow through. I don't recommend Kaldness. It would trap too much of the bacteria that dies and sluffs off.
This shows the inside of the 55 gallon drum with the 2" shower drain for the bottom drain. The reason it is positioned where it is, is because it is one of the lowest points in the barrel and it was the only smooth place on the bottom. Ideally it would be in the exact center but there is a mold seam there and it will not allow a good seal.
Here it is with the inlet. The 2" bulkhead fitting is the same as the one at the top left but it isn't visible in the picture due to the shadow. By installing the inlet like this it will slow the water down some and help with settlement. It also will be a support for the 24" "Matala Round" media I'll use first.
By installing the outlet like so the PVC pipe will serve to hold the media down and with the elbow turned up it will pick up oxygen before going back to the pond. When using it before a pump as in Dale and Patti's application you need to turn the elbow down and use a pipe strainer on the end.
This filter can be pump fed or gravity fed. With the 2" inlet and outlet a flow rate of 600-1,000 gph is recommended. If you gravity feed the filter with the pump after you will need to turn the elbow down so it doesn't pick up any air. The poly drums I use most have an inside diameter of 24" and the 24" Matala Rounds fit very nicely. I am using the green for the first stage of media.
I use a piece of egg crate to help hold the netting below the water with the outlet pipe over that. If you build one of these filters make sure any of the media you use can't get drawn into the outlet and clog it. If you have it outside cover the top of the barrel so leaves or other debris can't get in and stop up the outlet. Be sure to leave an air space between the lid and the barrel. I use heavy shade cloth to cover mine outside.