I had 2x250W on my 50G tank. No problems with heat at all. I did use 2 radioshack fans on the hood though.
Depends on what you want to keep. At the time I was interested in getting a maxima clam and had heard that 250 was better than 175 for the depth of the tank. Some say yes, some say no. A deal came up on teh 250s and I couldn't resist it
Christy
