I have found vinegar is usually too weak to show proper bubbling, so I use the cheap hydrochloric acid sold for pH adjustment in swimming pools. I can't remember whether it is 3 or 5 normal. Now that I can see fizzing on soluble minerals. [Wear eye and skin protection for it can burn.]
Just because it fizzes does not mean a rock is unusable. It just usually means it contains some calcium and/or magnesium carbonate, and will be slightly soluble in water with an acidic pH. As it dissolves, it adds both GH and KH, which is exactly what many of our Fp. and Nothos want. The higher tds seems to ward off velvet for them.
Testing for foaming just lets us be warned to watch the hardness and do regular water changes, so it doesn't get too bad. After my tds meter, I find my GH/KH test kit most useful (I'm on a well with no chlorine). My pH pen is my least-used test, most of the time.
Two things seem to stop the rock from dissolving. As the carbonate/bicarbonate in the water (KH) builds up, it tends to buffer the water up toward a pH of 8 and solubility drops to near zero. Also, an insoluble skin seems to form, over time, so initially fast-dissolving rock or even sea shell becomes less soluble in even acidic water.
This latter effect seems to make stuff collected from fresh-water sources generally pretty inert. Beach rocks and gravel, OTOH, have been only exposed to alkaline sea water, so they can be pretty active when first dropped into a tank.
I would avoid heavy-metal ores like lead, zinc, copper, etc. I grew up in the mining business so I can recognize many of them, but I despair trying to describe them as they are so highly varied in appearance. They are also unlikely to fizz under acid conditions, so that test is mostly good for spotting carbonates. A long soak in low-pH soft water will generally make them pretty safe, as long as you do reasonable water changes, too. BTW, some shale and dolomites never seem to become less active. IDK why.
My Daphnia tubs have a lot of pretty rocks soaking in them, awaiting a home in an aquarium.
Wright
01 760 872-3995
805 Valley West Circle
Bishop, CA 93514 USA
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