Even though she doesn't have any cracks? Or would it make Cracker sick?

Your snail is beautiful as he is! I would leave him alone. Heaven only knows what it might do to him and your fishies. I would say better safe than sorry! And I really do think he's gorgeous!

I only recomend painting for damaged snails.

Speaking of which, what is recommend when painting damaged shells? Clear nail polish work alright for that? Like the hardner type stuff....

Yup I use clear nail polish.

It's perfectly fine to use in the aquarium as long as you let it dry completely before you put the snail back in.

if hte shell is not damaged, theres really no use in painting...shells are pretty as they are, and even clear nailpolish clouds after awhile and starts to look crappy