It doesnt look like black beard algea to me. The light green spot def look like green spot algea. Are you using co2/ Ferts?
For h2o2 treatment ive done it for bba. Works very well. I havent done full tank treatment. I mixed about 10% peroxide 90% tap. I just took affected plants out and let sit for 20 mins in solution. A couple of my more sensitve plants leaves melted due to dip. But grew back very quick.