One bristlenose would be good for a 20 gallon tank, or like I said: a couple of otos. I hear they do well in schools, so if your tank isn't overstocked and you have enough space, get 4-6 otos. They should keep the tank clean. I will myself get about 4 of them for my 10 gallon tank since I am starting to get algal growth in there.
Emma, a pleco will eventually grow too large for a 10 gallon tank. I am not saying this to discourage you from having a pleco. But what if this fish grows big and will be hardly able to move in that 10G ? Are you planning on keeping the pleco in a 10G ? While your friend may me mad at you, I am sure she'll understand if you tell her about the nature and requirements of this fish. She wouldn't want the pleco to suffer, would she? Or maybe you're getting some bigger tank soon?
