Before doing eythromycin, which is a waste of 10$ unless you actually need the medication for you wish in my opinion. Go out to walmart or riteaid and buy yourself a bottle of Hydrogen Peroxide (H2O2) which go for around maybe a dollar for 350ml? Just turn off all flow and spot for and leave for 30 minutes at 1ml per gal. You could also use this time to spot dose all your other algae such as BBA. I left the lights off that day and turned back on the filter but you could probably turn the lights back on if you wanted too. The BGA turned brownish the next and is now totally gone from my tank.