Just wanted to let you know my experience with a new UV sterilizer. I purchased a 24 watt GKM (Green Killing Machine) regularly sold at Pets%$%$rt. I was fighting greenwater in my 75 galon planted tank to no avail, having tried a 4 day complete blackout and still failing to conquer this. I can only say that this device WORKS, and what I used to think was crystal clear water wasn't crystal clear at all!! Tank looks great and I've now addressed an issue that may have caused the problem to start with. (I raised my lights higher over the tank and shortened my photoperiod considerably. Just wanted to share.