Pebbles on a river bed. Reflections and ripples on the surface.

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. Rock composition could possibly change the hardness and pH of the water in a way that harms your fish. Collected outdoor stones may also be covered with contaminants that can affect aquarium water. 

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

How to Test Rocks

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

Testing for calcium can be as simple as placing a few drops of vinegar on the rock or gravel you are considering using. If the vinegar (an acidic substance) fizzes or foams on the rock, don't use it. The chemical reaction you are seeing indicates that the stone contains calcium.

Another way of testing rocks and gravel is to place the washed stones in a bucket, fully submerged in a small amount of the water you use in your aquarium. Test the pH and hardness initially, and then let the water with rocks sit for a week and test again. If there is a significant increase, these rocks or gravel will cause problems in your aquarium.

Rocks to Avoid, Rocks to Use

The rocks to avoid are those that are highly calcareous—meaning they have a large amount of calcium. Also, always avoid sharp-edged rocks that can harm your fish. 

Rocks to avoid include:

  • Shells or crushed coral (these are not ideal for most freshwater tanks, but may be used for African cichlid tanks, where higher pH and calcium hardness are desirable)
  • Limestone
  • Geodes
  • 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 and ground glass
  • Sandstone (always test before using, as it may contain traces of limestone)

Many gravels and rocks 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 be sure.

Where to Find Outdoor Rocks

Outdoor gravel and smoothed stones can be collected in the great outdoors. Try ocean beaches and lakefronts, in dry wash beds, or along the banks of streams and rivers. However, avoid collecting stones from underwater locations, especially in protected environments, as removing stones can disturb native habitats that wild fish and plant life depend on.

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

  • Landscape companies that sell smooth river rock and other aggregates
  • 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 rocks or gravel before using them in your aquarium.

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