Sump tanks are a solution to the problem of having exposed heaters, wires, etc. It's also provides a reef system with more water volume and better filtration since the detritus will settle in the sump tank rather than your display. I had this sump installed underneath my 240 for 2yrs. So to answer your question, there is no real answer, sumps are pretty much as big as u can go for the space u have available. My girlfriend has a 40 gallon sump connected to her 10gl nano.