For a 90 gallon you would want a few of the big boys, I think the rule of thumb is 40x your watever volume or something? So 3600 gallons per hr circulating, if that is the rule i cant remember. Someone will correct me. As for placement it really all depends on how your rock is setup to move around and through and all about it. Finding the sweet spots can be difficult and time consuming but well worth it. As for quantites of power heads, you could go 1 of two ways, 1. Get a couple more powerful ones to achieve the max circualtion, or 2. Buy more of the lesser powerful ones, i find option #2 would be more beneficial since you could place various poweheads all about your tank so to acheive no dead spots. Again this is just my opinion. Others will chime in. :) Good luck!