Head is a combination of vertical lift or height and losses due to pipe friction and fittings or direction changes and reductions in size.
A Mag 12 is way too big for a 30G system even if you restrict the flow. At 6 feet it will still be pumping in the neighborhood of 900 GPH. The wattage will be around 90-100 watts in doing so. Heat will be a big problem is such a small system.
Recirculating some of the flow back to the sump sounds good but is really counterproductive. Centrifugal pumps only draw the watts required to do the work at hand plus some for inefficiencies. By recirculating you are pumping closer to the full 1200 GPH so would consume around 110 watts and generate the 110 watts of heat. A better option, within reason, is to valve to discharge side of the pump so you reduce the output and reduce the wattage since the pumps is moving less water.
I say within reason because this pumps is just too big to do this efficiently for such small flow. You will never get it to operate efficiently because of its design capacity. A better option would be something more like a Mag 5 or Mag 7 although I am not a fan of Mag pumps in general, they are power hogs, run very hot compared to others and are noisy. Eheim and Ocean Runner are two much better choices more suited for what you need, maybe an Eheim 1250 or 1260 or an OR 2500. They are both very efficient, run much cooler and are silent. I tried Quiet Ones and had two literally burn up so will never go there again. Tried a Rio Hyper Flow and had the impeller magnet swell up after a few months so bad it would not turn in the housing, another write off. Had the same problem with a Via Aqua pump too.
I have 4 Ocean Runners in my systems today and some of them are over 5 years old and still chugging away with the original impellers and magnets.
