BN pleco's seldom grow much more than 4", sometimes 5" for a really large, old male. A 10 gallon is fine for young ones, a 20 long for a mated pair. Males are territorial, I wouldn't recommend keeping more than 1 male in less than a 4' tank. They are excellent algae and diatom cleaners, 1 adult will keep even a 55 quite clean. If there isn't much algae their diet needs to be supplemented with spirulina wafers, romaine lettuce, etc. They don't eat live plants, and are generally good community fish.