Testing rocks for carbonates
How to test for carbonates. Rocks with a calcium carbonate component — the "lime" in limestone — will raise the pH of the aquarium water. Unless you're looking for this effect, as in an aquarium of Rift Lake cichlids, you probably want to avoid it.
Everyone you ask will recommend the old trick of putting a teaspoon of vinegar on a suspect rock; if it fizzes it's going to raise the pH. That would be clear enough, but what if it doesn't produce a bubble? Some prep is necessary. PaulK posted at AquaLink, 13 Aug 2003: "The acid test would depend on the strength of the acid, but would also depend on the surface you are testing as well. In order to be accurate, a fresh surface should be tested. It is possible that the rock may have a carbonate residue, which would effervesce if acid is applied, though the actual rock itself may be something that contains no carbonates. On the other hand, a rock may be a carbonate and yet may not effervesce readily when acid is applied. Dolomite, for example, must be powdered to react with an acid."
Household vinegar has never worked for me. I only got vinegar to fizz when I poured it over some old mortar I picked out of a wall. A friend suggests that pickling vinegar is more acidic than the common salad dressing kind I was using. I might even recommend instead that you put a suspect rock in a bucket with enough water just to cover the rock. Test the pH now and test again after a week or so. If the pH hasn't noticably risen, figure that the rock is "aquarium safe." Does this sound too lackadaisical?
Muriatic acid. Some aquarists test using muriatic acid, which is a 20% solution of hydrochloric acid, available at your hardware. For heaven's sake be careful with this intensely caustic stuff.
Test kit acid. Here's a better idea: if you have a nitrate test kit, you may also already have a better test than vinegar for lime content in rocks. My Aquarium Pharmaceuticals Nitrate Test contains two bottles of prepared solutions. Solution #1, labelled "Caution: contains hydrochloric acid," comes in a handy squeeze dropper bottle. If a drop or two on a candidate rock fizzes, or even bubbles, that rock would raise the pH in the aquarium.
Another test. You can also check for effects on pH of a rock by testing the water of a plant nursery tank, placing the rock in the tank and re-testing after a month