Store credit is where the shop takes the fish and gives you some credit that can be used in their store.
eg: You take in 10 guppies and the shop gives you $1.00 credit for each fish. You now have $10.00 in credit at the store. It is not cash in the hand, but you can take $10.00 worth of goods from the shop in exchange for the fish. You can also use the $10.00 credit as part payment for a more expensive item. It is simply deducted from the price of the item/s you buy.
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Killifish aren't big sellers and neither are Paradise fish.
Guppies, platies, swordtails and angelfish are the biggest sellers.
If you have limited space and not many shops to sell to, you could look at breeding some of the dwarf cichlids or smaller rainbowfish. Apistogramma cacatuoides is a South American dwarf cichlid that will breed happily in a 2 foot long tank and you can usually sell them. Melanotaenia praecox is a small rainbowfish that also sells well.
If you are trying to make money from fish, set up some outdoor ponds and put the baby fish out there to grow during the warmer months. And have a number of tanks and tubs indoors and breed the common fish like guppies and platies, and have some of the more unusual stuff like rainbowfish, dwarf cichlids and killifish.