i have 3 rbp in 30 gallon tank, current sizes are 5-7 inches, when i recieved them 3 months ago they were only 2-3 inches. they are feed ALOT. with them in a smaller tank it will require more frequent cleanings to keep your nitrite and nitate levels, down. also have heard that pirahna may crack a glass aquarium due to swimming into it. good luck with them and maybe in the future you may want a bigger tank.