Planted Tank Guru
Join Date: Sep 2006
Location: Northern California
firefighter: If it's inside a canopy, your water temperature is going to shoot way up. It might not be that significant if the bulb is 12" off the water, though. Is the canopy vented? I know that for reef tanks, if the MH are mounted close, then you'd need a chiller to prevent the temperature from going up too much.
Another problem with going with one MH bulb versus two sets of T5s or whatnot is the lack of control of your photoperiod. I have 4 T5 bulbs, 2 are on for 10-12 hours a day, 2 are on for a midday burst of 3 hours to give 216 watts for just that 3-4 hours. With your 175w, you're going to have a constant ~6WPG the entire time the lights are on. Without CO2 injection and a religious fertilizing schedule, you're going to be fighting algae much more than you're going to be enjoying your tank. If possible, go for two 70 watt MH setups for better control of the lighting schedule, but since you already have the 175w, then I doubt it...
HybridS130: You can find used MH setups, 175w with XM 10,000 bulbs, IC ballast, and 18" spider reflectors for ~$175-200 shipped.