Just estimate from the information in the thread. The gist of it is T5HO bulbs very often come with very good reflectors and the plants cannot use it all so algae attacks.
Are you fertilizing the tank at all? Lots of bright tanks with pressurized gas need more phosphate and nitrate than fish poop provides and any tank will need some added potassium and trace elements. Another link for you.
And another because it is short and succinct.