There could be a number of things going on. The growth of excess algea is due to excess nutrients in the water. Most likely, PO4
. Get a phosphate test kit and see what your levels are at. Also, test the PO4
in your source water. If you are using tap water for top off and changes that could be the source. Evaluate your feeding schedule. How often and how much do you feed? What do you feed? Make sure you do not have any dead spots or spots without flow. finally, it may be time to change your bulbs. How old are they?