Hmm this sounds weird. From a quick peek at the fish minigame script, the way you get new fish is determined by the number of your female fish and tank state. If it's well maintained, then each female fish has up to 30% chance to produce an offspring.I don't sell any fish. They're dying around 10 - 12 each round. I'm getting less than 4 young ones each round. I'm over those fish at this point. They can all die in episode 12 for all I care. Those fish were a waste of money if you didn't cheat. I don't know what the benefit is of taking care of them at all.
A simple trick you can do is to input
ft_debug = True
in the dev console. This will add some under-the-hood data to your fish tank overview:
"ft repro" is the important bit, and the chance to get new fish.
edit: as for benefit, they're basically money print, given you can earn more money selling surplus fish than it costs to get them food and cleaning supply.