How to make rocks safe for aquarium fish

Updated July 19, 2017

Aquascaping your aquarium is a fun, exciting activity. Many aquarists find this to be one of the favourite parts of owning an aquarium. Live and plastic plants, driftwood, gravel and rocks are all items that can used to decorate aquariums for freshwater and saltwater fish. If you plan on using rocks in your aquarium, you must ensure that the rocks are safe for your aquarium fish and that the addition of the rocks will not change the chemical dynamics of the aquarium.

Choose smooth-edged, non-jagged aquarium-safe rocks from your rock source, whether an aquarium store, garden nursery or a location outdoors where collecting rocks is not prohibited. You should also check that the rock type you are purchasing is safe for the type of aquarium that you have or plan to create. Generally, granite, quartz and certain types of lava rock (basalt) are thought to be safest for aquarium use. All rocks need to be tested first, as rocks are often mixtures of minerals that may contain harmful pH-altering constituents.

Use the aquarium brush to scrub each rock under hot water. Do not clean with any chemicals as some rocks could absorb these chemicals, which may be released later in your aquarium tank. Be sure that all of the dirt, dust and other materials are scrubbed off of the rock.

Pour one to two drops of vinegar onto the rock. Watch the rock carefully to see if any bubbles form. If you do see bubbles, this means that there could be chalk present in the rock. If you have an aquarium that requires a lower pH environment, this rock type could be harmful for your tank as it could raise the pH dramatically. On the other hand, if you do not see any bubble formation then your rock has passed the first test. Repeat this step on all your rocks.

Fill your plastic container with the same water that you use to fill your aquarium (dechlorinated tap water). Use the directions that came with your pH test kit to test the pH of the water. Add the rocks that you plan to use in your aquarium to the filled container.

Let the plastic container sit for several weeks. During this time, you can keep using the pH test kit to check the pH. If after several weeks, there has been little to no change in the pH of the water, then your rocks are most likely safe for your aquarium in terms of pH. If the pH does rise dramatically, you will need to consider the type of tank and fish that would thrive in the environment the rocks will create.

Add the rocks to the aquarium if you have determined that they are safe for your aquarium's conditions. Continue monitoring the water chemistry frequently. Monitor fish health closely. If your fish become sluggish or are acting differently after the addition of the rocks, you should remove the rocks, complete a water change and evaluate more of the mineral components in the rocks you have obtained.


It would also be best to understand the mineral components of the rocks you added so you will know if you should monitor the water for certain compounds. For example, some rocks are iron-rich and aquarists who use iron-rich rocks have to monitor the iron concentrations of the water. Overall, pH is an important indicator of whether or not rocks are safe for aquarium use. Close monitoring of fish health after the addition of the rocks is also key.

Things You'll Need

  • Aquarium pH test kit
  • Aquarium brush
  • Vinegar
  • Aquarium-friendly rocks
  • Large plastic container
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About the Author

Laura Ross has been writing since 2003, with her articles now appearing on various websites. She obtained her Bachelor of Science in the biological sciences from Whittier College, a Master of Science in environmental sciences from the University of California-Riverside and is pursuing a Ph.D. in environmental sciences from the University of Arizona.