The short answer
Test them with vinegar. Drip a little on the rock: if it fizzes, the rock is calcareous — it will slowly dissolve and raise your water’s hardness and pH. No fizz usually means it’s inert and safe for any tank. Beyond that, scrub rocks clean without soap and avoid anything with metallic veins, rust or a chemical smell.
The vinegar test
Calcium-rich rocks like limestone react with acid. Put a few drops of ordinary white vinegar on a clean, dry spot of the rock and watch closely:
- Fizzing or bubbling: the rock is calcareous. It raises hardness (GH/KH) and pushes pH up over time.
- No reaction: the rock is most likely inert and won’t affect your water chemistry.
This doesn’t make a rock “good” or “bad” on its own — it tells you what it will do to your water.
Cleaning and what to avoid
Scrub rocks under running water with a stiff brush to remove grit and debris — never use soap or detergent, as residue is toxic to fish. Steer clear of rocks with metallic veins, rust-coloured staining or any chemical smell, and don’t collect from roadsides or industrial ground where they may carry contaminants.
Building your hardscape
Once you know your rocks are inert (or you’ve matched calcareous rock to hard-water fish), you can scape with confidence. See aquascaping for beginners for arranging stone with wood and plants, and how to prepare driftwood for the wood alongside it. To check how hardscape is affecting your water, use the water testing hub.