Step one: Talk to the owner about it. If he makes excuses and doesn't fix it, let him know that you're moving to step 2. If the threat doesn't work, make good on it.

Step two: Look up that store on yahoo and other review places, and leave a review explaining how poorly the animals are cared for, and the filthy conditions of the place. Include the fact that you tried to get the owner to correct the problem, and he refused.

Reputation is everything, and it has never been easier to let everyone know when something is wrong with an establishment.