the practice of using vinegar on rocks is from the aquarium world. it is a base test to check for alkalinity in rocks. some rocks, such such as limestone, would alter water chemistry, making it too alkaline (hard) in a closed environment. most animals prefer a more acidic (soft) water. a majority of plants prefer acidic environments as well. very few plants will survive in a hard water environment. as with anything, too much calcium can and will cause a calcium overdose in frogs. that is why we try to use only neutral rocks in our environments