Um.... RBP, meaning red.belly.piranha? If so.. I don't think that's going to work. You wouldn't be able to keep one in a 10 gallon for very long. They need lots of room to swim.
zero. You could keep three 3" rbp in a 55 for a little while, but that would even be a little tight, they're usually skittish fish and need room to move around otherwise they'll always be bouncing off of the glass.
If you are wanting to keep RBPs you will need to plan on having at least a 4 foot long by 18" wide 75 gallon tank. RBPs are schooling fish and should be in at least a group of three. Ideal would be to have 5-6 in a 6 foot long 125 gallon tank. With proper care RBPs will reasch 8-9 inches in length and can even grow bigger.
75 gallon minimum for 3 reds.....Get that poor fish out of that 10 gal. IMO NO FISH should be kept in a 10 gallon. Do research next time BEFORE you go buying up what you THINK you want to keep.
I know a guy who has deffinate 13" P. Nattereri (reds). And other species of Pygo can reach sizes of 24"(P.Piraya). Ternetzi, Cariba, and Piraya all grow, and act soooo much differant than reds.