I breed guppies.
It’s not really that hard, put some guppies together in a tank, and you will have fry usually before the week is out. The trick is in keeping the parents from eating them, and then keeping them well fed at the same time, then changing them into a new tank and raising them until they are big enough to sell.
I do it for a variety of reasons.
Having fish tanks in my office is very soothing. It helps me relax and creates a great atmosphere for writing. The sound of running water, the swirl of colours as they swim, the pleasant rise of air bubbles.
I can re-sell the babies for store credit at our pet store, which helps us offset the cost of keeping our menagerie.