Somebody I know opened an LFS
in Toronto in 2006. He started with mostly bread and butter fish- corys, tetras, loaches, plecos, barbs, but he also had a decent cichlid section. He did ok, but he was just scraping by. He wasn't making great profits.
Then he had the idea to become the only LFS
in the area that specializes in cichlids. He now does more business than just about any fish store or fish section in the city, has moved to a larger location, and draws in almost every cichlid enthusiast in the city due to his constant level of fish quality. He buys all his fish locally from local breeders. He only imports oddballs like rare cryptoheros and so forth.
My suggestion for your reptile store wanting to start a fish section is to reach out to the local fish keeping community and find out what the hobbyists are doing. Hobbyists buy sometimes hundreds of fish per year. Much better people to deal with than fly-by-night fish keepers, who will only buy fish once or twice.