Really , it depends on what kind of pet you wish to have for a fish. In my experience with working in a family pet store my Aunt owned, the best kind of fish to have for a child is a gold fish. You can put them in a fish bowl that doesn’t require any pumps for oxygen and it is easy to clean out when it is needed.
Then if you wish to have a step up from that , guppies and Mollies arethe next best species. Usually when you get alot of fish from them breeding, you find friends that want to buy them or you could sell them for a profit by putting out signs that says Fish for sale. What is your desire when you think about having a fish for a pet?