If you are serious about starting a fish farm, then you should likely select an species that survives in your particular area of the planet. In Canada, people farm salmon and trout. In Florida, people farm salt water varieties and coral.

Zat U Sam? It would be nice to make some money from our hobbies, but it aint that easy. The answer to what kind of fish, pretty much explains why it's not easy. For the small breeder to make any money at all, he has to specialize in a fish that is expensive. The fish is expensive because it is rare, it's rare because it is hard to breed in captivity. The easy to breed fish are inexpensive and it doesn't pay to raise them in a backyard situation. The commercial breeders can make a profit off of them because they are in mass production. They have hundreds of acres of ponds, they raise millions of fish, every thing that can be automated is automated, they have the shipping and handling facilities. There actual profit to the breeder on a fish you buy at the pet-shop for up to 5 bucks may be just a nickel for a dozen. The costs of shipping are what makes the retail price high, water is heavy and cost more to ship than the fish. The breeders also have a sales force and a market, the pet shop is not liable to buy anything from you, an unknown with no guarantee on your fish. They don't want to take a chance on introducing disease or parasites into their tanks. The more expensive harder to breed fish are raised indoors, where the temperature, pH, and all those conditions can be closely monitored. It can be extremely time consuming. However, if you do have the desire to go into the bizz, you might as well start breeding something just to learn the problems you'll face. Don't try to make a profit, concentrate on breeding and treat it as an extension of your hobby. Start with the real easy fish. Maybe even guppies ( you might even find a market as feeder guppies,for some of those), try some zebra danios ( maybe the same market as the guppies), try some goldfish ( maybe a market as feeders and bait) some of the smaller cichlids are easy to breed and they tend their brood which is just real interesting to see and they will enjoy the new born guppies. BUT.. if feeder guppies are selling retail at 1 dollar a dozen expect to get around a dime or less from a shop, maybe 25 cents direct.Your market is going to be direct sales to people who have some carnivorous fish, small Mom and Pop fish stores and bait shops that might buy just a few at a time from you . The problem is finding them. Is there an aquarium society or fish club in your area? Join , the other members may buy feeders from you, some of the chain pet-shops who do not sell feeders may allow you to put a notice on their bulletin board, sometimes there are public bulletin boards near the other shops, there are "want-ad "type news papers that run free ads. ( Shopper's News, Greensheet, etc.) Visit small pet shops and ask them if they will buy from you, visit small bait shops. Most people would say "I don't want to discourage you." But I DO want to, at least from expecting quick profits. Don't expect any profit. But do try some breeding just for the fun and education of it. And set yourself some goals. For example the first year breed and raise a spawning to full salable size, even if you don't sell any, ( for guppies that ranges from just born to full adult, depending on what's eating them) that way you'll see how much room, time etc. you really need to raise as opposed to just breed your product. As you gain experience, in a year or so, try some fish that retail from 10 to 20 bucks and try direct sales at half or less than the pet-shops get. After that keep moving up. Once you are proficient at goldfish maybe move up to koi. Who knows you may be the one who makes a fortune when you breed a fancy bottom feeding pond-scum sucker that becomes the new hot fish.

I agree with the answer from notmrjohn wholeheartedly but in addition to that I will tell you my thoughts and my experiences thus far. I too have decided to try my hand at breeding albeit indoors. I decided to start with mollies. I am breeding sailfins and dalmatians as I call them. I only get 60 cents per fish but it is worth it because it keeps me in fish food and filters etc. I usually keep about 30 bucks in credits at my fish store which is cool with me since I run 3 tanks. Spread your fins and sail away and see where the water takes you. You can never see where you will wind up if you dont jump in the water. Good luck to ya.