on the U.V killing good bacteria...if you add supplimental beneficial bacteria, its a good idea to turn off your U.V for a day or two. but good bacteria colonizes hard surfaces. when added, bottled bacterias are temporarily free floating until they find a home. during this short period is the only time that a U.V presents any danger to them. now anaerobic or bad bacteria remains free floating..killing it is a plus. U.Vs also kill pathogens,parasites,disease carrying bacteria and viruses...these benefits are on top of its intended use of killing green water (single cell) alge. that said, for its alge only intended purpose, turning it off once the water is clear is fine!...36 watts would be the smallest you'd want....check eBay for deals.