Hey Timmiegun, the way I see this is that it's not a concrete requirement, more of a player's choice thing. As you say it's a waste of space if you only want the bottom-dwelling creature in the tank but it's not necessary for that animal to be in a shallow tank. I wouldn't want to put an animal requirement in as that would remove the option to put the animal in a deep tank with others. See what I mean?

I think they look quite good in the 'lagoon' and 'u-shaped' tanks which are only 2 units deep.

Or perhaps what you're really after is a very large shallow tank? I'm sure a modder would be happy to oblige you! ;)

I see what you mean, and am working on a large shallow tank. But I mostly meant a requirement that modders could use to force some species into shallow waters. (An example is the crocodile that is currently on the workshop or the soon to be released mangrove items.
But I see where its coming from. Thanks anyways!

Ah, now you've explained a bit more that makes sense. I can see the need for something like this for a crocodile which likes to swim near the surface in shallow water.