I built a fish room years ago, it is being upgraded to a fish house. I found most shops only give you tokens not hard currency. Maybe I bred the wrong fish.
I usually swapped my fry for more fish or pots of food. I think that is how it goes in the UK.
It was a lot of fun seeing the little babies, seemingly come from nowhere, and learning the different methods for some of the different species. I got some great fish for nothing, and the ones I already had ate well. I used some fry to feed my bichir, this was how I weeded out the runts. If you're going large scale I think you may need a big boot or a piscivore. Every new shoal has a runt or two. Shops do not like receiving feeder fish as part of the deal.