I'm currently using a Maxi-Jet 900 and a Mini-Jet 606 in my 20G. According to the specs for these pumps that should give me about 380gph. However, I'm not too happy with the current flow in my system. I'm planning on getting either a Maxi-Jet 900 (or 1200) to replace the Mini-Jet. I think that the higher flow rate would be helpful, but mainly I like the fact that I can aim the output of either Maxi-Jet any way I want. I may even keep the Mini-Jet in there somewhere.
A lot depends on the rock work and what you plan to keep in the tank. I have a 20 g. cube with a couple rio 600's that's been going strong for several years. But the Rock Work and Corals are very open and water moves around the entire tank freely.
I believe it was Matrin Moe at IMAC last year that suggested using wieghted strips if small bubble packing material to check water flow. Work well and gives you a realistic idea of the flow patterns and amount of movement in your tank. I'd suggest checking the patterns and actual movement. Then decide on what's the best way to proceed. Sometimes a couple small power heads are better than a single large one depending on the tank.
