Sounds about right to me. The pump for tha 75 gal aquarium prob. hooks up to 1/2" line easy and It would depend on the pump rate(gph) if that was to small or restricting for the flow. You need to circulate the whole 75 gal. through the growbed at least once an hour on flood and drain. I'm guessing even if you run continuosly that your pump is prob. not too fast for 1/2" pipe, 1" would be great to dump the gb back into the sump and then 1/2" back to the tank would keep good head pressure on the pump. Just make sure you check your pumps max lifting height. In other words if your pump can only lift water vertically for 5 feet and your tank is 7 feet off the floor, problem!
