Color, growth, and depth in the tank, would be a few. I am going to try the XM and see about it, but I am totally not liking the hamilton bulbs, I will say however the hamiltons were used when I bought the kit. May or may not make a diffence, thats why I said I was going to swap soon to XM-10k also will be replacing the actinics in the fixture as well and see how that turns out. But not wanting to get into a arguement on the lights only gave my honest assessment.