I'm sorry Ralok, I'm no real programmer. I didn't know how to help you. I know, at one time, they exchanged guests for animals, but I wouldn't know how they did it.
As for the aquarium, you mean, that you place it in a tank and animals go in it?
If so, I think you should probably program it as a shelter for animals.
The ZKlounge has a shelter tutorial;
you can find it here.For underwater shelters, it's probably somewhat different, I would advice to unrar the original file that contains the shelter, take out a marine shelte .ucs file and compare what you should change your .ucs file to.
(First make the item as a decoration in Ape, then you can still change the .ucs file.)