I don't know much about the commercial ones available now because the last one I used was one I built myself for treating seawater for a cold water marine setup. The water is usually in contact with the bulb so the bulb and the water need to be kept clean or there will be not enough penetration of UV, and the water needs to be passing through it at the rate specified or there will be not enough contact time. I would suspect you are overfeeding and providing food for the algae. The only tanks I have that "problem" in are my turtle tanks because they are such heavy and dirty feeders. I use the green water to culture daphnia so it is not a problem. It would take a very fine filter to remove it (like a diatom-- down below one micron) and it would clog quickly.