Do you want to dim the light because of algae? Poor plant response in some way? In general, T5HO can't be dimmed. However, there are ways to fine tune.
Do you have one or two switches? Some of the 4 bulb units allow you to run two or four bulbs at once.
I have a pair of two bulb fixtures. To fine tune lighting I have two bulbs come on first, then all of the bulbs together, then two bulbs again. In addition I even have some low wattage LED's just for viewing in the evening and early morning. The only time all the T5HO bulbs are on is during the middle of the day.
It's not because the bulbs are too bright, but just to fine tune/control the amount of light my plants need and are getting.
You could also raise the light, so it is further away. And in a more drastic move, remove some or all of the reflectors. Really depends how much co2 you are dosing and the ferts you are supplying. If you are injecting at something like 30ppm co2 and dosing EI, you need a pretty good deal of light.
