55 gal.
2, 175 watt MH Blueline 10,000+K w/electronic ballasts, and 2, 40 watt 50/50 flourescents.
Hey Hondo! Using Metal halides in a 6" hood would heat the water considerably, but could be done with enough air flow, however, the evaporation would be tremedous IMO. Also, I would worry about water hitting the hot lamps and breaking them, and this could hurt your pocketbook!
Jay1