Thanks for the reply Dave. I tried to increase the photoperiod from 6 to 8 hours and the algae started reappearing again (not very much, but still...). I put the photoperiod at 7 hours to see if that will work. However, although algae is always in the water, I think there must be something wrong with the system. I think that without something being wrong with my system, I would be able to increase the photoperiod more. I think there has to be something like too much dissolved organic matter or not enough NPK, which I don't dose, or something that's making it hard for the plants to grow and easier for the algae to take over. Anyway, that's just my take on it. See you.