It's a lot harder. The exhibits themselves aren't the hard part to change, it's the animals inside them. Currently the exhibit animals are rooted to one spot, only moving on a set animation in that one fixed position. They only change places when you leave and come back but never actually move around the exhibit. That can't work with fish, as fish are constantly in motion.
Not necessarily, everything is a static animation loop in exhibits, each animal had to originally have their biome made, then the parts made to add to the exhibit, then then animals added. Fish would just be another animation loop within the exhibit. Animations like fish can be made with a simple movement animation, and then attached to what you can essentially call a track that the animation will follow, and then bake the two animations together. Then you would just place that asset inside the new exhibit, which would still be rooted to one spot, just the anchor origin would be in the center instead of 2-3 locations.
In terms of complexity, it would be much harder to create a new animal for a habitat than a static animal for an exhibit.
For something like an eel, octopus, stonefish, firefish, Blenny, or mantisshrimp, they would behave the same as reptiles where their animation loop would be to sit in one place in the rockwork or coral, as realistically these animals are sedentary, spending a majority of their time just outside of their home range or moving only to hunt.
a good example of making a simple fish animation with a track can be seen here -
Source: https://youtu.be/0s-GqRFCHsQ