wow a mag12? do you have a sump refugium or a simple wet/dry, or is there just a sump with a skimmer and some reactors, unless its a wet dry i would not use a return pump that large, there is no need for it and the turn over will be to quick for a refugium,skimmer or media reactors to do a proper job.
another thing i would not use your return pump to generate flow, use powerheads or a closed loop pump with return heads spread around the tank