As for the melting the acrylic, you will be boiling eggs in the water long before the acrylic melts.
With a 175W over a 25 gallon, i would say you need to start out at about 10 to 12 inches, with atleast 1 fan blowing directly across the bulb. Keep a close eye on your temp and be prepared to raise it a few more inches, or add another fan if needed.
It is all worth it though, the MH is bar none the best light and the best thing you will ever do for your reef. Remember to slowly acclimate your tank to this new light over the coarse of about a month, so you don't burn everything up. Start out only running it about 2 hrs per day, and add about 20 to 30 minutes every other day until you are at your normal light schedual. If you go too fast with it, all of your corals will bleach and die. You are going from having a light over your tank, to your own personal sun. Even though the MH wost likely won't seem as bright to your eyes, it is very direct intense light.
