I have a fluval 206 (200lph) on my 90L and have great flow, I was running two powerheads as well but it was blowing the tetras all over the place. Ideally I would have a fluval 306 or 406, but the flow is getting CO2 to all parts of the tank. I would suggest a contingency plan of buying one big filter to start with and having funds in reserve incase you need to add more filtration. People are not wrong with 10 times the flow but for me it would be like a washing machine, i don't think my tetras would like it.