I've read all the replies and see ppl suggesting big $ fish which would be a good idea except that there is a large overhead associated with these fish. Between the cost if food and heating and space required and how delicate these fish are and the fact that they are so expensive and delicate that you're cutting out most of the population that is keeping fish they there's little to no $ to be made.
I'm going to stay firm on my fist suggestion of high quality koi. No heating, food is cheap, they will thrive in just about any water parameters, they spawn like crazy, and they are cheap enough that just about anyone can afford them.
Sent from my mind to my fingers to my phone to the MFK servers to you