I have a 150 and I run it at around ~900gph or more (I'd have to check the pump box to be exact). Anyway, one overflow doesn't keep up unless I were to run it deep which defeats the purpose, so I use Two 1'' overflows.
Yes, but for one, I don't like PVC over 1" on my tanks. It's too big and then behind the tank you need that much more room between the tank and the wall.
Second, it's just better to run two overflows. It makes better water movement, and I enjoy the symmetry.
The amount of water going through the overflow depends on the level of the water in the tank. It's just like a siphon, the higher the water level in relation to it's destination, the stronger the flow. Anyway, The water would have to be like 2 inches above the opening of the Overflow for one of my overflows by itself to keep up in my pump. So I could "run it deep" and just have the water level be 2 inches above the opening, or I could just use two overflows like I do. The reason not to go deep is because in the event of a power outage too much water would drain from the tank and overflow my sump. Get it?
