Can I Use My Own Outdoor Gravel or Rocks in an Aquarium?

Pebbles on a river bed. Reflections and ripples on the surface.
Mint Images/Steve Prezant/Getty Images

Using your own outdoor gravel or rocks in an aquarium can feel like a good way to save money, and the act of rock-collecting has its own pleasures. An aquarium takes on a custom charm if its bottom is covered with stones you hand-picked yourself. However, using your own gravel or stones in the bottom of the aquarium comes with some inherent risks if you don't first test the stones to make sure their composition will not change the hardness and pH of the water in a way that harms your fish. Collected outdoor stones may also include other contaminants that can affect aquarium water and harm fish. 

Experts have mixed opinions on the matter, but many argue that unless you are an expert at identifying rock composition, it's best to go to a pet shop and purchase use rocks and substrates that have been deemed safe for aquarium use. Other authorities, however, believe that using your own gravel and stones is acceptable, provided you follow instructions on how to test them to rule out hazardous materials.

How to Test

The principal danger to using your own outdoor gravel and stones in an aquarium is the possibility that they contain calcium, which that can change the pH of aquarium water. First, make sure to wash the stones thoroughly to remove all loose grit and contaminants.

Testing can be as simple as placing a few drops of vinegar on the rock or gravel you are considering using. If the vinegar (a base substance) fizzes or foams, don't use it, as the reaction you are seeing indicates that the stones contain calcium.

Another way of testing rocks and gravel is to place the washed stones in a bucket of the same water that you use in your aquarium. Test the pH and hardness, and then let the water sit for a week and test again. If there is a significant change, the rocks or gravel are likely to cause problems if used in your aquarium.

illustration of how to test aquarium rocks
Illustration: Wenjia Tang. © The Spruce, 2018

Rocks to Use, Rocks to Avoid

The rocks that you should avoid are those that are highly calcareous—meaning they have a large amount of calcium. 

Rocks to avoid include:

  • Limestone
  • Geodes
  • Shells or crushed coral (not ideal for most freshwater tanks, but may be used for African cichlid tanks, where higher pH and hardness is desirable)
  • Marble
  • Dolomite

Safer rocks include: 

  • Granite
  • Quartz
  • Slate
  • Lava rock (take sharp edges into account, particularly with fish that have sensitive barbels, such as the Cory species)
  • Onyx
  • Sandstone (should always test before using, as they may contain traces of limestone)

Remember that a great many gravels and stone will have a mixture of minerals, even in the same stone. Even if you think you have correctly identified a stone as a safe mineral, always test to make sure. And always avoid sharp-edged rocks that can harm your fish. 

Where to Find Outdoor Rocks

Outdoor gravel and stones can be collected yourself in the great outdoors—from beaches along the ocean and lakes, from dry wash beds, or from the banks of streams and rivers. Avoid collecting stones from the bottoms of active rivers and streams in wild, protected environments, however, as removing stones can disturb native habitats that fish and plant life depend on.

You can also purchase outdoor stones from a variety of sources:

  • Landscape companies, which may sell smooth river rock and other aggregates in bulk
  • Garden centers and nurseries
  • Home improvement centers with gardening departments

All of these sources can provide you with inexpensive and attractive rocks and gravel. Just remember to take care in making your selections, and always test your rocks or gravel before you using them in an aquarium. Happy rock collecting!