Uberhoust
Fish Herder
HCL is pretty easy to obtain as Muriatic Acid. You can pick it up at most hardware stores. At full strength it can burn your skin but the major issue is if you spill it, it can corrode other things, like your garage floor, some metals, etc. Muriatic acid when used full strength on calcium carbonate, pure limestone, will actively bubble, but dolomite, more of a Magnesium Carbonate, will bubble but not as actively, any bubbling indicates carbonates. I will have to try vinegar, to see how it works on some of my samples.
Finally even though a rock has carbonate minerals it may not react fast enough with the aquarium water to be an issue. The surface area of the stone has an affect, pure limestone will react faster in most cases than marble simply because the texture of the rock has more spaces, voids, and protuberances than marble increasing its surface area (same mineral in both). I use a pure marble rock in one of my tanks that with my water change schedule raises the pH at most by .2, ie it changes my usually 6.4 pH water to 6.6 pH, but it reacts very strongly with HCL.
Most rocks I have seen for sale in the LFS are not calcareous so I expect that at least some are tested and chosen correctly, but keep in mind that saltwater (I have no knowledge here other than very general) has different restrictions on the rocks, limestone might not have much of an effect on the pH of a saltwater tank.
I normally agree with @Colin_T, but there are a number of rocks that can contain minerals that affect the pH, including just about any sedimentary rock because many of them have calcareous cements, as well some metamorphic rocks can as well. Shales in particular often have a calcite cement. Obviously a lot of the rock is OK for the aquarium, including sedimentary ones, but in my opinion all should be tested unless you know your rocks and minerals. Additional some rocks have minerals that can decrease the pH, often these are metamorphic rocks with metallic minerals such as the various pyrites, and in some areas this type of mineralization is very common. Most igneous rocks do not have calcite except in rare cases but altered igneous rocks such as basalt collected near areas with a history of hydrothermal activity can have deposits of calcite, this is a common type of alteration.
Finally even though a rock has carbonate minerals it may not react fast enough with the aquarium water to be an issue. The surface area of the stone has an affect, pure limestone will react faster in most cases than marble simply because the texture of the rock has more spaces, voids, and protuberances than marble increasing its surface area (same mineral in both). I use a pure marble rock in one of my tanks that with my water change schedule raises the pH at most by .2, ie it changes my usually 6.4 pH water to 6.6 pH, but it reacts very strongly with HCL.
Most rocks I have seen for sale in the LFS are not calcareous so I expect that at least some are tested and chosen correctly, but keep in mind that saltwater (I have no knowledge here other than very general) has different restrictions on the rocks, limestone might not have much of an effect on the pH of a saltwater tank.
