You don't say if your 90 gal is a reef tank or "fish only" tank. The idea water turn over rate to strive for is 6-10 times your tank size per hour. Many aquarists feel that more is better, especially for a reef tank system. In your case with a 90 gal tank the flow rate to strive for would be 900 gph. The next question you have to ask yourself is where will the return pump be located. Location will play an important part in determining the size of pump to purchase. For example, if your return pump is located directly below your tank and is only having to pump 'up hill' a couple of feet than you can get by with a smaller pump than say myself, whose retun pump is in my basement. The 'up hill' resistence to the pump is "head pressure". So in theory (sp?) if you have a pump located 3' foot below the output level of your return piping you would look for a pump with a gph rating of at least 900gph @ 3' of head. That said, if you then looked at say a Mag12 pump, you would find that they have a rating of 1100 gph at 4' of head which would suit your needs. Hope all of that made sense. Sometimes I can be pretty winded and end up saying nothing.
The gph flow of your hang on overflow would depend on the size of your outlet hoses. I believe that most dual have a 1" outlet which would be a rating of 1200 gph for the dual outlets providing you do not reduce them going into the pump.



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