Rocks safe for aquarium use: the vinegar test, which rocks to use and which to avoid

Most rocks are safe for an aquarium. The real question is whether a rock will change your water: rocks containing calcium carbonate (limestone, marble, shells, coral) slowly raise pH and hardness. That's good for African cichlids and livebearers but wrong for soft-water fish like tetras, discus and many shrimp.

The quick check is a few drops of vinegar: fizzing means calcium carbonate. Then look for metal veins, scrub the rock clean, and make sure you're allowed to take it if you found it outdoors.

What you need

White vinegar (or, more reliably, a stronger acid such as hydrochloric "pool" acid, used with gloves and eye protection)
A stiff brush and running water (no soap)
A bucket and a pH and KH/GH test kit for a soak test
A fridge magnet

Step by step

Scrub the rock under running water with a stiff brush to remove soil, moss and loose bits. Let it dry.
Do the fizz test. Drip a little vinegar on a clean, dry spot and on any cracks. Watch closely, ideally with a magnifier. A stream of small bubbles means calcium carbonate. Vinegar is a weak acid, so the fizz can be slow and faint. Some experts point out that a stronger acid gives a much clearer answer. Rinse the rock afterwards.
Look for metal. Shiny gold, silver or copper-coloured veins, bright green or blue crusts (copper minerals such as malachite and azurite), or heavy rust streaks are warning signs. Try the magnet: a strong pull means iron-rich rock.
Check the edges. Run your hand over the rock. Very sharp edges and points can tear delicate fins and the soft bellies of bottom-dwellers such as loaches and corydoras. File them down or put them where fish won't brush against them.
Do the drop test on store-bought rock. Put a drop of water on it. If it beads up, the rock may be coated in oil or sealant. Scrub it, rinse it very well, or don't use it.
Soak test if you keep soft-water fish or shrimp. Test pH and KH of a bucket of water, put the rock in, and retest after two days and again after a week. A rise of more than about 0.2–0.3 pH, or rising KH or GH, means the rock is leaching minerals.
Add it to the tank and keep an eye on pH for the first few weeks.

How to identify a problem rock

pH creeping up week by week after adding a new rock, especially in soft water. See pH problems.
White crust forming around the waterline faster than before (more hardness).
Shrimp or snails dying soon after adding a rock with metallic veins (see copper).
Fins torn on fish that rest near the rock.

Safe rocks and rocks to avoid

Usually safe and inert	Raises pH and hardness (fine for hard-water fish)	Avoid or use with care
Slate, quartz and quartzite, granite (in most cases), basalt, lava rock, river pebbles that don't fizz, dragon stone, petrified wood	Limestone, marble, travertine, tufa, Texas holey rock, ocean rock, coral, seashells, aragonite, crushed coral. Seiryu stone often fizzes too.	Rocks with copper minerals (green or blue crusts), shiny metallic ore veins, rocks with oily or painted coatings, crumbly rocks that turn to mud, rocks from mine waste or industrial sites

Experts don't fully agree on some rocks. One detailed hobby reference considers pyrite ("fool's gold") harmless, while others say avoid it because it can break down and acidify water. When in doubt, leave a rock out, or soak-test it first.

Seashells and coral

Shells and coral are calcium carbonate. In a soft-water tank they slowly dissolve and push pH up, usually to around 7.6–7.9, and raise hardness. That's welcome in a Lake Malawi, Lake Tanganyika or livebearer tank, and some keepers use crushed coral on purpose to stop pH crashes. It's a bad idea with soft-water fish or with Caridina shrimp. To adjust pH deliberately, see adjusting pH.

Rocks collected outside

Is it legal? Collecting rocks is generally banned in national parks (in Canada and the US), many provincial, state and local parks, nature reserves and beaches. Some public lands allow small amounts for personal use. On private land, ask the owner. Check the rules before you pick anything up.
Is it good for the place? Rocks in streams and on shores are homes for insects, crayfish and fish eggs. Take a few small rocks from a dry bank rather than turning over a stream bed.
Hitchhikers: wet rocks and mud from lakes and streams can carry snails, leeches, parasites and invasive species. Dry them completely, or scrub them and let them dry for several days, before use.
Pollution: avoid rocks from roadsides, farm fields, golf courses, mine areas or anywhere that may have been sprayed. Experts disagree on how long garden pesticides last on rock, so when in doubt, collect from somewhere clean.
Cleaning: scrub with water only. Many keepers skip bleach because residue is hard to rinse out of porous rock; if you do use it, rinse very thoroughly and soak in dechlorinated water. Don't boil large or wet rocks: trapped water can make them crack.

Common mistakes

Putting shells or limestone in a tank of soft-water fish, then fighting a rising pH.
Scrubbing rocks with soap or household cleaners. See soap in the tank.
Stacking heavy rocks directly on bare glass. Put them on the bottom glass first, cushioned by a sheet of egg-crate or foam if possible, and bury the base in substrate, so fish can't dig them loose and they can't crack the glass (see cracked tank).
Building unstable piles. Glue stacks with aquarium-safe silicone or wedge them so burrowing fish can't topple them.

Products that help

pH and KH/GH test kit: shows whether a rock is changing your water.
Stiff brush: soap-free cleaning.
Aquarium-safe silicone or rock glue: makes stacks stable.
Egg-crate light diffuser: spreads the weight of heavy rock on the tank floor.
Crushed coral or aragonite: if you want to raise and steady pH on purpose for hard-water fish.

Why it matters

Rock chemistry decides your water chemistry. Calcium carbonate dissolves slowly in water, releasing calcium and carbonate, which raise hardness and buffer pH upward. Inert rocks like quartz and slate barely dissolve at all, so they leave your water as it is.

When to get help

Ask your local fish store or aquarium club if you can't identify a rock, if pH keeps rising and you don't know why, or if invertebrates die after adding new rock.

In the game

Multiple Tank Syndrome doesn't have a rock card yet. The Hard-water 40 and Shell beach 30 tanks show the same idea: limestone and shells make high-pH water that suits African cichlids.