Is this ok???

[brewercm]

Put a ball valve on the outlet in case you have to throttle it back and you'll be OK.

[witecap4u]

The mag 7 is proably a little big for that overflow, unless you throttle it back a little. That said, I think both are quite small for that size tank.

cs

[NaCl_H2O]

Are you sure about the 250GPH rate for Dan's overflow, that seems VERY low for just about any overflow? That aside, I agree the MAG7 is a prettly small pump for a return on a 120g (unless you are adding closed loops or powerheads) for adequate turn over. Assuming this is a Reef tank and not a FO or FOWLR?

If you have say 3' of head pressure from your return to the tank, the MAG7 will only perform at about 500GPH, which is about a 4X turn over rate. You want to target 10-20x total turn over (sump + powerheads, etc). Everyone has a different opinion on the 10-20x number, some even think 25 is better, but I think al would agree 4X is too low for a ref tank.

[Instar]

That small amount of overflow/return for a 120 is ok so long as you have other circulation that also provides aeration without introducing bubbles to suppliment it.