I think the total amount of water in the tank needs to be turned over by the fiter 5-7 times a hour.So for a 20g tank you would need:
20x5=100 gph
20x7=140 gph
So you would need a filter that rates between 100 and 140 gph.
My 55 gallon tank has a filter for 110 gallon tanks lol Aquaclear 110. I doubled it because I have a turtle and they make a mess.. But I was thinking if I should buy another smaller kind of filter (Possibly an Eheim) now that I got a PLeco who is a super-pooper.