for all my smaller tanks, I basically stick to x3 rule of thumb, 20 gallon tank gets 60 gallon filter + undergravel filter. 29 gallon tank gets 90 gallon filter + undergravel filter. I have only been in the fish hobby for a little over a year, so I havent had any problems with any of the filters I have gotten yet, but I have learned I dont like any of the on the back filters that have the little paddle wheels that turn with the waterflow, I prefer the ones that have the scrubber insert that you just rinse out occasionally. Additionally, I run an air pump into the power heads (I only use the Marineland 1140 powerhead -300 gph) for extra oxygenation. I change all insert filters on my smaller tanks on the first of each month.