Hmm. I think it's a bit like "realism" in a novel. By definition, you've thrown "real" out of the window because it's fiction, but the sub-reality within the novel needs to seem real to maintain the suspension of disbelief (Tolkien's particularly good on this if you read his writings on the subject). Similarly within the glass box, which is of course not natural, many aquarists, me included, like to create a natural looking environment, one which is believable. Plants, rocks, gravel and bogwood are believably natural. Plastic castles and galleons are not.You threw natural out the window when you decided to keep fish in a glass box
The fish don't care tuppence, of course. It's all about what the aquarist wants from his tanks.