To get the water out of the tank you need to use an overflow. You can buy or make a HOB model or you can fabricate one to go into your tank and seal it to the tank and the water will flow into there and out a pvc pipe down to your filter. As for getting it back into your tank for the sump version just get a submersible pump and put it in the last compartment of the sump and run a hose back over the top of the tank. As far as a DIY canister I don't know forsure but I'd think you could drill a hole in the bottom of the container you are making your filter out of and pipe it to an external pump and then back up to your tank. Do a search in google or on this board and you will find some very helpful diagrams and directions out there. HTH
