To actually make a profit you have to go large scale and even then it's tough. Hobbyist breeders are usually just offsetting some of the cost of their hobby.
I was just having a convo with a friend that money made is just money reinvested in my tank again. For example, sold some tahitian moon sand this morning and will use the money to buy more sand lol. Selling a light fixture and will probably spend the money on a sack of massivore.