I have had fish for many years...no means an expert but have never had this sort of problem before. My 5 gallon tank is constantly covered in a brownish/black algae. The first 4 years, no problems and suddenly last yr, it began. I have thoroughly cleaned it, have even used bleach and let it sit empty for months and the stuff still comes back after about 3 days. I have thrown all decorations away and bought new and switched out the gravel..even tried glass rocks on the bottom. The only thing I have never replaced is the filter unit. I can't keep doing a water change every 2 days. I have tested the water before the algae appears and after and everything is normal or safe...nitrites, nitrates, hardness, alkalinity and ph. I am on well water if that makes any difference. I am really at a loss and short of throwing away the entire tank and all accessories and starting over, I dont know what to do. Any help would be thoroughly appreciated.