If it has the All Glass top which consists of 2 pieces of glass with a plastic hinge and a plastic strip across the back, you can easily cut it to fit whatever filter you decide on. For a 20g tank, I'd probably go with a Penguin 170. My second choice would be a Whisper 30 although you could probably get by just fine with the Whisper 20. I'd rather oversize the filtration a little. I like the Penguin because it has the Bio Wheel for biological filtration. I believe this is the most efficient biofilter you can get for a FW
tank. Your bio filter is not disturbed when you change filter cartridges. Marineland makes a Penguin 125 also. I've had two of those and didn't like them...they seemed to clog up very quickly for some reason. The 170 is a good filter though and will run for years. I'm not sure of the measurements on these filters though. Another option, if you're willing to spend the extra money, is a canister filter. You can't go wrong with one of the smaller Ehiems.
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