Planted Tank Guru
Join Date: Jul 2010
Location: British Columbia, Canada
It is BGA.
Spot dose it at close range (with the filter off & water level lowered) with common household hydrogen peroxide (H202), using a plastic syringe or test kit pipette. It will bubble up and disappear overnite. Then you'll need to step up your tank cleansing routine (vacs) and increase/improve your water circulation within the tank to eliminate 'dead spots' - I use a small water circulation pump for this - a Hydor Koralia does an excellent job.