Daphne, Nitrate 0 means green water for sure in high light, CO2 suplemented tank. You have MH, substrate heating, Co2 with controler, what about ading some KNO3 ? You can get it as All green Stump remover at your local hardware store. Here you can get crystal clear information about how to dose it: http://www.actwin.com/fish/aquatic-plants/month.9704/msg00070.html Cosmin