I have plenty of stuff laying around to try a diy canister filter. One question that i have seen several places is head pressure to pump up to tank. Water seeks its own level, so if it is a closed system, would the pump (power head) just be moving water as it is in an under gravel system? My Grandfather had a gravity filter on a 55 back in the eighties with two inputs and one output. Once he got the siphon going it would run on its own with no pumps. So a 1" overflow with 1/2" return should work. I am using dollar store plastic scrubbies for media. I plan to use a 170gph powerhead. First try will be on my ten gallon then this spring will be a pond or bigger tank.