Good question. Always depends on the setup. I went oversized to work with the exposure time and that I wanted all the water to be treated.will keep that in mind. Right now running more for algae but can slow pump down when I add new fish. Not sure what the pump should be at!
I was using a smaller 40w with sump return and it just wasn't cutting it (heavy stocking doesn't help either). Went next size up and things are going swimmingly now.