Hello!
I'm not totally sure. But If I remember well, the fish can be found in debug mode, but they are also visual effects (VFX).
So even if you change the mesh in the Studio, their "original mesh" won't be changed since the effect will keep the original.
But you can override/recolor the fish though (without changing the mesh).
The fish effects are basically called "gp02_aquarium_(FISH SPECIES)". Or "ep04_aquarium_pets_(FISH SPECIES)".
Effect list.
You'd probably have to work between Sims 4 Studio and
VFX Tool. I wish I could help you, but I still don't know how to do it.
The fish you placed in game (that didn't move) is probably considered by the game as an ingredient and/or collectable item.
And the ones that move are probably just visual effects. Two separate things.
That's probably why it doesn't work or show up.
It's just my take on it. You can try to mess around a little more to discover more about how it works.
Good luck in your projects