Many times when light is present and there is no Chlorine or Chloramine in the water, Algae will grow. The best method to eliminate this is to remove all the old filters. Then run a weak bleach solution through your system, leaving it in there for 20 minutes or more is recommended. After the time has passed, rinse THOROUGHLY and wipe down this unit as best you can. Install new filters, and you should be fine. Keeping your system out of direct light should eliminate the chance of another algae bloom.