I had one lone Harley that somehow got into the bag when I got some other fish from the LFS. He lived healthily for 6 months without a problem but BOY, you should have seen it when I decided to get 6 more. Don't know if he was happy before but he sure is now!
I've had them for years now and they are quite big. They are IME quite easy to keep so if you had a couple die you may have a water issue so my advice would be to check that out and when you are happy then get some more to keep it company.