I'm not going to get into the ethical arguments about dolphins and orca, so we'll leave that aside and say it is what it is.
With fish and sharks and the like, I'm not convinced there's a place for them in
Planet Zoo. I often feel like people are looking back at
Zoo Tycoon and the way tanks worked there, and it just isn't realistic. Fish in captivity can't just be thrown into an open-top tank in the middle of a forest and be expected to thrive. There are a thousand reasons as to why, but water chemistry and temperature are the two big ones. This is why the vast majority of aquariums are indoors; much easier to maintain strict levels of chemistry without worrying about external temperatures, rain, air pollution, and so on.
We know that Frontier takes these factors seriously, which is why the exhibits likely exist the way they do. They knew it wouldn't be realistic to keep a lot of those animals in big open-air habitats, especially amphibians and invertebrates. The exhibits are obviously not perfect - they went with a one-size-fits-all system, so it can't be perfect - but it shows the intent. Maintain a regular temperature and humidity level for these animals to thrive.
So, exhibit-like aquariums are good in my opinion. You'd have similar sliders, but in this case to simplify things I think it would be one for water temperature, and one for salinity (some fish are freshwater, some saltwater, some prefer brackish in-between mixes). The challenge there would be compensating for the way fish move, as most fish species do move constantly, even if only a little. It's doable though.
However, big habitat fish are more questionable. Yes, they could simply add new water treatment facilities to manage the mentioned factors, and rely on us to build realistically or unrealistically as we please by using building pieces and keeping our tanks mostly indoors, but given the way they've done things so far I'm not sure that they would. So is there any other way to do this justice, and is it worth it considering that aquaria are not hugely common in zoos?
This isn't a question of Frontier's ability to make great things happen, either. I'm sure the designs they come up with would look fantastic, and I'm sure they
could conceivably come up with a solution to the various problems that doesn't amount to a bunch of Redditors shouting that "it's just a game, stop taking it so seriously". It's more a question of whether I think it's worth the effort.
So, to summarise again, aquariums like exhibits = yes. Happy with that, and the majority of zoos that keep aquaria tend to have these smaller fish in smaller displays because it's cheaper and easier to manage in a zoo context (there are obviously exceptions, but these tend to be bigger budget, world-class zoos anyway). Habitat fish = no; I don't see it as being worth the effort for something that needs to be pretty complex.