It basically doesn't matter what sized pump you get, as long as it's less than the rated gph
of the overflow. I understand it as if as if the pump slows down for some reason, less water gets pumped back up, so the water level is lower >> less water drained through the overflow. That said, you need to pay attention to head height, and also... don't go thinking that I meant for you to get like a 120 gph
pump :P. I'm unsure on saltwater requirements, but I'm guessing 1200ish or so would be good.
I forget, but for the pvc
type overflows, there needs to be a siphon breaker in case the power goes out or you'll end up with like 50 gallons of water on the floor