Remove the animal, and use white vinegar to get rid of water stains. Also, the Mr. Clean Magic Erasor is truly amazing. Don't forget to rinse in the same hard water, lol!
I agree with Marcia, use white vinegar. Soak the tank with white vinegar and let it sit for about 10 minutes, good ol'fashion elbow grease to srub it off. And then RINSE IT THROUGHLY!!!