I just got the Lifegurad 25w for a 90 gal. I was told that was just right. What really matters is the pump rating. The water has to move through the UV slow enough to kill all the bugs. The slower the H2O moves the more effective. Good Luck.
I was going to get the U2, but I have to wait like a week bacause CSL is upgrading it. The guy from the place that I'm getting it from said that the new U2 is going to be like a double helix or something. The 15 watt is going to 18 watts. Is this too much or just right? The guy said it was fine.