hey, sounds good ... aluminum ignites at over 1300 degrees F and melts around 1200, so you'd need more than a few MH
bulbs to cause any problems
but if you manage to start the aluminum on fire and add some water, you'll have yourself a big explosion as a chemical reaction between burning aluminum and water releases hydrogen gas!
on a more serious note, an open frame skeleton to hold the reflector will work, but will it be comfortable to work in the tank room with the lights on? perhaps some sort of wrap around or shield to absorb the spill off light, to keep it from blinding whoever is in the room?