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.
MY 10 GALLON PLANTED TANK:
2 x 10W CFL; Fluval U2 internal filter; MGOCPM/black sand cap
A Silent Cycled Tank