My mindset is this: it's preferable but unrealistic for Frontier to produce every species the community wants. So if they cover all of the bases with rigging and animation (for modding purposes) that could potentially suit a large number of animals (like one shark species, one octopus species, one small fish, one ray species, etc.) then I'm happy with that. Seeing as this could also be somewhat limiting, I would also like to float the idea of an animal maker's toolkit and a theme maker's toolkit for Planet Zoo- essentially giving official support to modding will extend the lifetime of the game exponentially in the absence of new DLCs. I don't see them stopping any time soon but if they decided it was no longer profitable or that the developers were getting bored with it this would be a good way of keeping things going for a VERY long time.
I don't know enough about what kinds of fish are common in zoos and whatnot so I'll just stick to fish I've personally encountered in zoos and aquariums. Habitat animals:
1. Sandbar Shark
2. West Indian Manatee
3. Round Stingray
4. Green Sea Turtle
5. Scalloped Hammerhead Shark
6. American Brown Pelican
7. Arapaima
8. Red-Bellied Pacu
9. Cownose Ray
10. Marine Iguana
Though the possible number of small fish is pretty much infinite, definitely larger than possible number of terrestrial exhibit creatures, the exhibits:
11. Clownfish
12. Giant Octopus
13. Blue Tang
14. Lionfish
15. Panda Angelfish
16. Moray Eel
17. Spotted Garden Eel
18. Some kind of crab
I will probably push the fish thing until the day they drop the game because I do consider it essentially important to zoos, perhaps even more than flying birds. Number one, there is no mechanical difference for exhibit aquatics, and secondly habitat aquatics would only require a way for keepers to enter tank habitats and some additional enrichment items, and feeders and coral for aquatic animals like sharks. That's all I can really imagine there being to it. I can see there potentially being an issue in that many exhibit aquarium species are kept together, but they solved that for the butterflies so I'm sure it won't be a terrible issue. It would also be ideal if they gave options for significantly smaller exhibit sizes with different shapes as well, and I'm sure most players would agree that the exhibits needed an overhaul yesterday, but that is a different discussion. To sum up my feelings on the matter, personally it's really disappointing to see Frontier every DLC continue to add more and more terrestrial animals that do little to change the experience of the game (though they are awesome and do add a lot of variety) when they could spend that time working on things that would be literal game changers, like aviaries and aquariums. I do think they'll get around to it at the end of the day, but it's kinda concerning how far they're into this without any fish. I guess it might just be my expectations that are causing the disconnect but the first few DLCs felt really essential when it came to experience changing animals and I figured they would continue with that as it went on- I love the SEA pack but I think that marked the point when I realized that it's less about fundamentally adding more to the game and more about just adding new content generally, which is not the way I would prefer for things to go.