Short of turning your fish collection into a "pay for view" area for your friends and neighbors
[emoji38], the only way I've known to make "real" money doing fish is to make a lot of them and sell a lot of them. This is a tough thing to do when you are tank space limited. What you then do is try to breed fish that will at least pay for themselves by breeding and selling the young. The fish that can do this are ones that are good parents and care for their fry so you only need 2 tanks ( 1 for spawning in and 1 for growing out.) Fish that come to mind for this, but are not limited to, are Kribensis, Brichardis, Rams ( sometimes), Plecos, etc. You can also do some cory cats in just 2 tanks but more is better. Raising common fish will not get you high returns on your time or investment. Depending on where you are at, talk to your local pet shop and see what they are interested in buying and breed those if they fit into the above criteria. Specialty fish make more money but if you can't sell them at the local level, you have additional costs involved which can eat heavily into your profits. Nothing beats the ability to walk into a store with a 5 gal bucket with a portable air pump filled with fish for sale. It's the cheapest way to move fish, but only when it's local.
Then there is the plants route. If you have the space, propagate some unusual or more desirable types.
Things to keep in mind are, if you have a number of tanks, buy in larger quantity. Buy the larger size can of food and store it in the fridge or freezer. Buy the bulk packs of filter cartridges instead of the ones and twos because they are cheaper. Better yet, make your own.
Yes, these things cost more to start with but are actually cheaper in the long run. Just figure out cost per ounce for food and price per cartridge for filters and you may be surprised.
Hope this helps
