I set up a 20 gallon guppy tank for my wife, filtered by a Cascade 500 canister. I wanted a filter guard, but something less restrictive than fine mess or sponge. I found an old 35mm film container and drilled it full of 5/64 holes (and I did the calculation to make sure the area of the small holes exceeded the area of the filter intake, so no restriction). The lid needs a little more Dremel finessing to fit over the intake before I give it a test run.
