Hmm, with a 20 long, you have about a million options for filtration. Since the tank is 36", I'd personally look at a very small canister filter, so you can take water in one end and output it at the other end, ensuring excellent circulation. You can also traditionally adjust the flow on them.
I have not tried this canister, or read many reviews (I will research if you're interested since researching equipment is a favorite pasttime,) but you might look at this little number:
Ok, I'm going to go research it anyway, since I can't help myself, lol. Ok, here seems a good review thread.
I'm going to look at the mentioned ZooMed canisters, but I think they're a lot more expensive? We'll see...