Using Outdoor Gravel or Rocks in an Aquarium

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

Experts and fish enthusiasts alike are divided using outdoor gravel in your aquarium. So don't be surprised if you see opposing perspectives on this prospect. There are several reasons for this. One is that outdoor rocks can have a variety of contaminants that may not be obvious to the naked eye, but which can be hazardous to your fish.

Another more common reason is that some rocks or gravel are composed of materials that can cause a change in the pH and hardness of your aquarium water, which in turn can be harmful to your fish.

Some people feel that unless you are an expert at identifying rock composition, it is best to go to a pet shop and purchase use rocks and substrates that have been deemed safe for aquarium use. Others will describe how to test rocks and gravel to rule out hazardous materials.

Using outdoor rocks and gravel is great, but only if you are willing to test them. If you don't intend to test your rocks and gravel, it is best to purchase them from a pet shop. But be aware that on occasion even materials sold in pet shops have been known to be hazardous.

How to Test

Testing can be as simple as placing a few drops of vinegar on the rock or gravel you are considering using. If it fizzes or foams, don't use it, as it contains calcium deposits. Another way of testing rocks and gravel is to place them in a bucket of the same water that you use in your aquarium. Test the pH and hardness, and then let it sit for a week and test again.

If there is a significant change, the rocks or gravel are likely to cause problems.

What Can and Cannot Be Used

Wondering what is safe to use in an aquarium, and what isn't? The rocks that you should avoid are those that are highly calcareous – meaning they have a large amount of calcium. Calcareous rocks and gravel will alter the hardness and pH of your water.

Rocks to Avoid Include

  • Limestone - very high in calcium carbonate
  • Marble - has similar effect as crushed coral
  • Geodes
  • Dolomite - often found in crushed coral
  • Shells or crushed coral - not ideal for most freshwater tanks, but is used for African Cichlid tanks, where higher pH and hardness is desirable

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)

Regardless of the type of rock or gravel, you should test them to be sure. Also, take care to choose rocks that have smooth edges, as sharp-edged rocks can harm your fish.  

Where to Get Outdoor Rocks

Obviously, the great outdoors is a prime source for rocks, stones, gravel, and sand. However, it's also possible to purchase rocks and substrates from places other than a pet shop. Places they can be procured from include:

  • Outdoors – avoid riverbeds, as you can disturb the native habitat
  • Garden nurseries
  • Gardening centers at department stores
  • Landscaping companies

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 the aquarium.

Happy rock collecting!