Unpainted smaller ones are safe for larger tanks with below ideal pH as they release calcium salt into the water which causes a pH spike. Larger shells are safe for salt water aquariums and huge aquariums. The largest shell the sell at PetSmart (xx large which is about the size of my fist) is downright dangerous for small tanks, especially for mine which has only a 1.4 water volume and practually no buffers. I can't tell you how high it went as my tester only goes so high, but I can tell you this. It was high enough to make my fingers tingle/burn when I had to stick my hand in there! AND I had to take out 4 litters (3 scooped out and 1 gravel) and squeeze out/soak my Marimo Ball before I got a pH that was wnl!
Got a new "cave" thanks to Walmart clearance. A clear orange pumpkin shaped tealite holder. Soaked it in vinigar water twice (it originally had black paint on it I had to remove), then boiling water, then reg water overnight and testeded its pH before I put it in the tank. They love it!