Is that big enough of a tank? I read they get around 6-1/2" long? Plus, a lot of areas around the country won't let you have pirahnas. For example, I know they aren't legal in Florida to have out of fear of people releasing them into local waters. Not sure about the Wimple variety though.
I don't think a 20 gallon tank would be large enough for one, and they are too nervous when kept alone. Wimples are scale eaters, and can get picky about food. They will get the scales off of even fairly fast species. I had two with a trio of large Rainbowfish, and the Rainbows were denuded overnight.