Vanilla 3D modeled fish and Squid do not spawn underwater, this is for 1.16.4. I have searched far and wide in the lukewarm, warm, and in general just oceans. Turtles do spawn however. I am also just wondering what set difficulty it should be.

Are you saying that both Vanilla and Aquaculture fish do not spawn, or just Aquaculture fish?

I have not seen any aquaculture fish except the swamp turtle spawn. Basically the only thing in the water is drowned and mobs like that. land animals do show up.

Are they more rare on hard mode?

I think I fixed it, it was a mob spawning rule

They're not more rare depending on difficulty.
So vanilla fish are now spawning either?