Can I keep one red belly pirana in a 20 gal tank and it live for many years

I do believe that we have some people on this site who are really knowledgeable about the care and maintenance of various kinds of fish. However, most of them might not be currently logged onto the site. In the meantime, you might want to click on the following search until someone comes along to address your question personally.

Thanks!

Red Belly Piranha Care Tank - Google Search

OK sounds good thanks

From what I've been reading about these fish, everything says that yes
A 20 gallon would be fine.

I have no experience with these fish because they are illegal to keep where I live, however it's been my experience that when you have a fish that can reach 11 - 12 inches in length, a 55 - 75 gallon should be the minimum.

Good luck