May I suggest an Iwaki pump? Their magnetic drive models pack some serious power at a price you can find for much, much lower than a March pump. They are rated to pump corrosive chemicals at temperature extremes, and do have a duty cycle of 100%, so can run constantly. If you know anyone with an aquarium, chances are they will know about Iwakis as well, and are generally considered to be the premier choice in that field. I would recommend an MD-10, -15, or -20, which can usually be found on ebay for ~$60. I personally snagged an MD-30 (complete overkill) for a cooling project of mine at $45 shipped. Good search terms are Iwaki, magnetic drive, MD, and pump. Using the "search title and description" checkbox really helps, too!
Only downsides are that like the March, it's not self-priming, and you will need to regulate flow!