Did you manage to isolate the ballast to reduce the vibrations that were causing the 'humming' noise?

BTW, in reality you aren't overlighting your 20g, a 250w Iwasaki at 12" water depth would be comparable in Lux to natural sunlight at 15-20' NSW, give or take a bit. I have all the Lux values for the 'sakis around here somewhere, I'll try to dig them all up and put together a chart we can all reference.

So your microalgae problem will eventually balance itself out as the system matures and the nutrient uptake is increased, though a skimmer of some sort would help remove the excess nutrients a little quicker.