Willowspirit
Crowing
Baby chicks being raised by a momma hen in a free range situation would've eaten a lot more than fish. Yes, you can feed them bits of fish, kitchen scraps, etc. The caveat is to do it in moderation.
Edit to add: "Sand" is not grit. Get them some chick grit.
I understand your initial concept, but where do mama hens get proper chick grit to go with the other crud they're eating? I think you have the rest of their lives to vary from a good solid basis in chick crumble.