I'd suggest a simple bowl and betta on the front with the question "Happy betta?"
Then on the back a far more dramatic drawing of the betta punching/bursting through the fish bowl glass and the exclamation 'NO!'
A little cartoonie, but dynamic and it'll make people think about it.
I think this is a great idea btw - and I think for teh sites...well, the one that sells teh shirt (if not your own site) might want to be on it. Or if you sell it through art sites (say Deviant Art). Not sure how that all works out tho.