my tank is next to a window, but the blind is always closed, and if i have the door shut to the room, it is virtually pitch black. so i'd like to think that isn't the issue, it certainly didn't affect my other tank, which was probably in direct sunlight for a quarter of the day! so i'm quite confused.Any sunlight reach your tank? Noticed my tank in summer when it got to much sunlight would get the green water bloom. Blocked the sunlight and it stopped.
Could be your lighting time and intensity as well.
my lighting is the aquasky moon, the 50w version. i run it 6 hours, from 9am until 3pm, but when the tank was not suffering from green water, i had it running probably about 10 hours a day.