While it wouldn't make much sense to use cards with fish in a tank ...

How much more sense does it make to use cards for actual combat? :P

I don't think that the cards are intended to be actually present on the battlefield: they're part of the player's experience, not the character's, so I don't think that they'd be objectionable in such a setting as yours. If you do want to justify them, however, simply note that they're laminated, or made of plastic.