one of those nail brushes you can buy from the supermarket, and a toothbrush for any nooks and crannies just make sure to do it in a bucket of old tank water and keep the brush just for fishtank stuff
it being just for fish is for the fishes well-being not other peoples. Don't think it'd make any difference to someone's nail cleanliness, but introducing stuff to the tank from a dirty brush would be bad...