I have an "El Natural" Walstad type tank and for the last month I have had a bad problem with green mat (I am assuming is cyanobacteria) algae. It is covering the substate and plants. I keep trying to remove it but it grows back very quickly. The tank has been up and running almost 2 years with no prior problems. It is a 75 gallon tank with 7 medium to small fish (2 sunnies and the rest are minnows/shiners). It is planted with local plants (mostly watercress and ludwigia) from local lakes. I have 1 emporer filter for circulation with just some batting for mechanical filtration. The bio wheels are removed. With this filter the circulation is definetly better on one side of the tank. The only change which I am guessing may be part of the problem is my (2) 65 watt power compact hood went bad and due to financial restraints I replaced it with (2) flourescent shop lights (about 2 weeks before the problem began) with a total of (4) 40 watt bulbs (2 cool whites and 2 soft whites). I have always had the lights on for 11 hours and am continuing the same with the new lights. Any suggestions would be greatly appreciated.