The Sydney Zoo in Western Sydney has no aviaries, neither does Taronga Western Plains Zoo in Dubbo, these two are in Australia
While I have never visited the Sydney zoo, as it is a fairly new addition to the zoo world, there is plenty of information available on it, to deduct that the lack of aviaries is directly related to its construction budget. The zoo has some sprawling habitats with plenty of ground for some of its residents, however the exhibit layout is quite simplistic and basic in most areas. Aviaries for the most part can run up a construction budget rather quickly. Sure you can built lower cost aviaries that do not necessarily eat a massive chunk of the budget, but this does not seem to be the case with Sydney. The whole construction of the Zoo was tagged at around 40 million (at least that is what is available online) most single state of the art zoological exhibits featuring aviaries cost a lot more than that to built, so you can do the math.
There is also Taronga Zoo Sydney, and its proximity to Sydney Zoo making it difficult for them to compete. For me it was an odd choice building such a budget zoo just under 35 miles from one of the world's greatest zoos (a personal favorite of mine) not sure that budget aviaries would have made any difference here.
As far as Taronga Western Plains Zoo, during my visit I remember seeing plenty of bird species, specially native ones. Many of the birds I remember seeing where in open air exhibits and often part of mixed species habitats. The presence of birds in a zoo is not limited to aviaries, and with the safari style layout of Taronga Western Plains Zoo it would make sense that their birds are showcased in a different way.
It really changes nothing, you will encounter far more aviaries in zoos than annexed aquariums. Birds however are integrated in many different ways, not exclusive to aviaries. In any case I would love to see a few aquatic species in Planet Zoo that can complement the zoological collection.
African Cichlids would be a fantastic addition for Hippo exhibits. Arapaima and the massive Arrau Turtles would be a welcomed addition to Amazon inspired areas. Piranha would be great for a standalone exhibit animal. Koi fish and Asian River Turtle species would be amazing for recreating lush Asian gardens with ponds (often common in zoos with thematic Asian exhibits) In short, fish are definitely a possibility, and one that should be explored by Frontier, just not in the scale you suggested unless is a stand alone project, and definitely not taking priority over other basic elements and species needed to complete zoos in general.
Now, this is a far fetched idea, and will probably never happen, but given the success of the game, and the inclination of so many players to all things aquatic, there are enough reasons for Frontier to work on either a Planet Aquarium game or large scale aquatic DLC that can be introduced at a later time. Based on what I managed to understand about how these games are created, it would take some time and a sizeable team to pull it off, but it is definitely something they can manage if they wanted to. Could Planet Zoo with all of its already present technical limitations handle such an amount of content? That remains to be seen, but it would probably sell very good if they did.