I started raising fancy guppies when I was a kid and was always curious about raising piranhas.
In 2008 I order 20 one-inch babies from a guy on eBay. They grew fast eating little chunks of meat and fish.
I thought I would raise these up and sell a few to help pay for the remaining ones. What I found is that there is no resale market for piranhas no matter how nice they are. People will come and eagerly look at them but don't want to take them home.
I ended up putting them in a big 300 gallon tub in the garden. It's fun to tell people you can rinse your hands there but don't dangle them for too long because we have big piranhas in there.
I ran an ad to sell the ever growing fish - one guy asked how many would it take to fill up a moat? He is tired of trespassers and wants to dig a moat and put in piranhas to solve the problem. I didn't hear back from him.
The fish are easy to keep, just don't over crowd them or over feed them. The water can get dirty and then they die. I got an over sized biofilter that worked well. I fed catfish chunks (cheaper!) and raw peeled shrimp till it got expensive to feed 20 six inch hungry fish. I eventually got a minnow trap and would catch a bunch of live minnows from the creek for free each morning.
One reason I am getting rid of the tropical fish is that the fluorescent lights use a lot of electricity (40w x 2 each) plus the heaters can be 300 watts in a 55 gallon fish tank. I had 5 big aquariums and it could suck up the power. Not a good idea if you want to run with the least amount of solar panels. Also the lights had a very high EMF radiation which we were able to check with a $40 EMF meter from EBay.