I bought 5 red belly pirahnas at the same time, only 3 are growing why?
Do They eat their own?
Welcome to Fishlore!
Are they in the 20 gallon in your aquarium info? Then they aren't growing because the tank is very overstocked. 1 pirahna grows huge and needs a 55 gallon tank minimum, 5 need a 180 gallon tank or so. Here's the care sheet for them: https://www.fishlore.com/Profiles-Piranha.htm
I suggest you hurry and either rehome them to a larger aquarium or give them away, as a 20 gallon tank is far too small.
Do you know about the nitrogen cycle? Click this link, it is important reading. Also, may I ask if your tank is cycled? This water chemistry is essential to success in fishkeeping.