This is usually a sign that the tank has got out of balance for some reason. Has something changed recently? A new food, new person feeding the fish, light tubes replaced, an unusual water change from the normal, new fish or plants added (or removed)?
To reassure you to start with - the fish will not be bothered by the green at all, it just looks ugly.
I would do a 3 day blackout, rather than just 48 hours. I also assume you did not feed the fish during the blackout? I'm not actually sure if the blackout trick works on green water. I have only used it on cyanobacter which is the horrible sheets of green slime that grows over gravel and plants.
Sometimes you never figure out what caused it and often, if you are lucky, it will clear up again as fast and mysteriously as it appeared!
I get green algae blooms when I change the light tubes :roll: