Random thought:
This game could be greatly improved by 3 different kinds of aquarium exhibits:
- Tropical Coral Reef
- Amazonian Rainforest (Piranha)
- East Africas 3 Great Lakes
These kinds of aquaria are really common in zoos and with just these 3 types most needs would be fullfilled.
Besides the current shape a round one and multiple different water (and preferably also exhibit) heights would be appreciated.
For the fish species itself, imo it would be best to handle them like enritchment items that you can toggle in execution with them looping around certain spaces with different loops, like a few for ground dwelling, some for "free" swimming and a few specialised ones like around an anemone for clownfish.
For each aquaria having an assortment of 6+ options of fish would be great, with some like Piranha being unable to share their aquarium and others being able to be kept together.
This way with just 3 different "bases" we would have many different options for our aquaria just by mixing different fish, toggleable items and what ever we place into them on other scenery items, making it imo a quite elegant solution to introduce fish in a costeffective way to the game.
It wouldnt be perfect ofcourse, but it would be good enough and be an interesting new take for an exhibit animal.