You put fish outside in ponds/ tubs to breed during spring, summer and autumn. Then sell anything you don't want to keep, and bring the remaining fish inside and keep them in holding tanks over winter.
For fish like livebearers, buy 4 or 5 female guppies, platies and swordtails, and put them in separate ponds (1 pond for guppies, 1 pond for platies, 1 pond for swordtails). Add a few aquatic plants and let them breed. If you feed the adult females well, they won't eat the young and at the end of summer you will have hundreds of fish to sell. You can also sell off unwanted plants for more money.
You should deworm and treat all new livebearers for gill flukes before putting them out in ponds.
If you have spare ponds then add 1 or 2 prs of rainbowfish (don't mix species). Add a heap of plants and let them breed. When you see babies (usually 1-2 weeks after adding the adults), move the adults into another pond and rear up the babies in the pond they are in.
For gouramis and Bettas, add 1pr to a pond/ plastic storage container and put some floating plants in. Feed well and let them breed.
For dwarf cichlids, add 1pr to a pond and have some caves and plants in there. Let them breed and sell the babies when 2-3 months old.
If you have 10 containers producing fish over spring, summer and autumn, you will probably get 2-3 batches of fry from each pr of fish. Each batch can contain several hundred young that can be sold when 1-2 inches long.
If done properly, you can end up with several thousand fish to sell and once they have been sold, you can put the money into your bank account.
One more time @Colin_T ... I. DONT. HAVE. THE. SPACE.... I cannot keep another aquarium inside of our home... We just cant. So I can't do holding tanks for the winter. But I can just sell them to my LPS and then restart in the spring and summer