imo, red belly piranhas are easy to keep as long as you keep them feed, have proper filtration and don't jam them in too small a tank.
imo, Red Belly Piranha are considered advanced, even when housed properly in a giant tank. Sure, baby anything in a 20 is easy, but it is irresponsible to say that a 20g for a Piranha is ok. We should be encouraging the OP to research this species, not give a general green flag to a potentially nasty end. A singular baby Piranha will not be happy and healthy by itself in a twenty, period. If you have to keep this fish/s in a small tank when they are very small, spring the extra cash and put them in a 50.
Good luck to you OP. I love black sand.