i would say...
mag 12 is too big as a return pump for your system. a reef ready 90 only has one overflow (unless modified or not the standard 90). that 1" bulkhead can handle about 600 gph draining. the mag 12 obviously puts out twice the amount the overflow can handle. not to mention, if you are pushing too much flow through your sump, your filtration will not be as effective. how big is your sump?
on my setup, i have a 180 gallon tank and a 70 gallon sump. my return pump is only a mag 9.5. if you are looking for tank circulation, i would suggest you get more powerheads...like the koralia powerheads. the two maxi jet 1200's only put out 295 gph each...so you don't have enough flow.