How to refer to the 'sections' of an aquarium?

Solution 1:

Both of your examples at the end are fine, but I think the word you might be looking for is tank, which fits perfectly in all three of your constructions at the beginning of your question.

If you want to be consistent, (though you don't need to be in this case), you'd need to reform your later sentences to

The people are looking into the tank.

and

There are many people at this tank.

Solution 2:

What about "exhibit"? Or are you talking about different sections of the same tank?

Solution 3:

I do believe that tank is a great answer, water tank to be more precise (there are gas tanks too). The glass you are looking through is called glass window (don't have idea why). But when you go to see different species of marine life you go to place with different maricultures.

Solution 4:

I don't recall aquariums using the word "tank" or "water tank". I believe the word of choice is "habitat".