Convicts are easy breeders but are all but worthless for resale. LFS are flooded with them from unwanted spawnings in home aquariums and they are not worth anything to serious hobbyists.
Zebra plecos are high dollar value but they have small spawns (5-10 fry) and take more than a year to grow to sellable size. You will need to invest as much as $1000 to get a breeding group and may make that back in 2-3 years. But that doesn't count the food and electricity in between so it won't fund itself ever.
Guppies are too common and although you can produce a lot of them it takes several months to grow them to sellable size and you need to carefully cull them to make sure you are only selling the best of the best. At most they are only worth about $.50 to $1 wholesale.
BN plecos are the best income generating pleco as most LFS will take them at a reasonable price ($2-$5) depending on size and they have frequent and large broods. But keep in mind that it takes 8-10 months of growth to get them to sellable size.
Unless you have a lot of tanks and can produce a lot of fish with little overhead, and have a market for them (local stores, a fish wholesaler, good club auction and swap scene) you should not expect the hobby to pay for itself.
IME the best revenue generating fish are Yellow Labs, Ps. demasoni, and Albino Eureka peacocks. They breed easily, grow quickly, and have good color at small size which makes them attractive to dealers. There is always a market for them with retailers and hobbyists as well.
but you should do it for fun not to make money.
Andy
