I helped a friend of mine with the exact same problem. His tank was two years old with a ton of algae and after this "amazing cure" his tank cleared up in less than a month.
Stop using water from Wally Mart, or Safeways, or Co Op. Unless you know exactly how often they change their filters then the chances are you are getting more than good RO water. Go fork out the money and buy yourself your own system. That way you can keep track and change filters accordingly.
That's my two cents worth tonight. Hope it helps
