How long is the tank? It all depends on what you have inside the tank. Depends on your rock work and sand. Every tank is different so what works for you may not work for someone else. Also, flow is very subjective so what may seem like a lot of flow for you is probably not enough for someone else. The only thing we can recommend is to test and test until you find something that works for your tank. Usually an XF250 on a 90 is fine but again, all depends on the tank itself.