Also, how long has the tank been set up? The initial establishment of nitrifying bacteria is really only the first part of a long series of cycles a new tank goes through. Most go through a series of algal blooms (somewhat of misnomer since it's not all strictly algae) -- brown (diatoms), red (cyanobacteria), green -- as the various equilibria become established. It can easily take six months or so for everything to settle down. As recommended above, nutrient control, proper lighting (no "old" bulbs) and water flow are paramount, but if the tank is new be patient; it's normal and will go away on its own provided your other factors are kept in check.