We have used the Magnum in the past and was very disappointed in its performance. They are problematic, debris ends up in the socket where the impeller sits and grinds away at the housing/ impeller. When the impeller gets scored enough, the magnet comes in contact with the saltwater. It will rust and swell causing the problem to worsen rapidly as well as leaching minerals from the magnet into your tank. The replacement impellers are expensive and you can't repair the scoring on the housing without replacing it. Provided that you have the space, I would recommend getting a small (5-10 gal.) tank and a small pump. This will be cheaper and give you better long term results in my opinion. HTH
BTW, I have some extra chaeto if you need some. I will be at Oceans Floor tomorrow for the frag swap, let me know and I can bring some for you.