The rule of thumb that gets tossed out alot is 1
the tank capcity.
So the 90 gal
tank would be 900GPH flow. Now this might seem like alot but in reality its no where close to the flow that a natural reef can experence. As strictly marine said if you go with a combo refuge/sump then you might want your flow thru the sump to be lower than this. Maybe 5-7X vs 1
. You can then use powerheads in the display to boost the overall current in the main tank or you coudl put a closed circulation system on the tank so as you have it pushing water in thru the main tank. Since its closed you have the benifit of keeping your tank powerhead free.
I have approx 12X flow thru my 80 and really think I need some additional current in some locations.