Maybe try a different BN pleco? I have a brown one, only 2" long, and he has eaten almost every bit of algae in my 125 gal discus tank in about 2 weeks. The wood and live plants were covered and they are all bare now, he is working on the glass and filtering equipment now. Of course, I siphon a large amount of algaepoo, as I call it, from the tank bottom every evening. He is an eating machine!
Barb