Usually a "tax break" -- i.e., being able to claim an expense against a business's taxable income -- depends on the business being able to demonstrate that the item is essential to its work. So if your company has an aquarium where the fish tank was an essential part of the decor, say in a seafood restaurant, that would be something you could claim in the same way as you would claim any other part of the decor, like the chairs or carpets. But if your boss just wants a fishtank in his office instead of the usual pot plant and photos of the kids, that isn't something essential and so claiming it would be less clear cut.
Cheers,
Neale