How long are your lights on for currently. How old are the bulbs? Are you dosing ferts? Co2? Any algae eating fauna currently? RO/DI or tap for water changes?
I'm glad I found this thread, because I just started having the same problem. My lights are 2x24 watter T5s, on for 12 hours a day, the bulbs are about 8 months old, I dose a capful of Excel every 3 days, and no algae-eating fauna. I use tap water for water changes. I did a 20% change today. What should I do?