It all depneds on flow through the tank. WHat do you have for a filter? skimmer? Maxi makes a small powerhead that I use in my 72 for the tight spots. Depending on the livestock you might need one anyways.
I run two maxijet 600s in my 28 gallon, a 250gph h.o.t. filter/fuge, and a skimmer that's run by a maxijet 1200. Mine is a reef as opposed to a fish only tank, and you can get away with somewhat less flow in a fish only.
For filtration and flow rate 10-20x and hour is standard. so if you have a pump that has 200 gallons per hour, on a 20 gallon tank that means your turn over rate would be 10x an hour, get it?