Try to avoid purchasing lava rocks with too many pointy edges because they can be dangerous for your fish. If your aquarium mostly has small fish, they can easily be injured by the sharp edges while they are trying to make caves. The size of the rock does not matter— but the shape does.
What rocks are toxic to 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.
Is lava rock toxic?
Not all "lava rock" is created equal. Some can contain tons of iron and other metals that may be toxic. For the most part, though, the stuff you can get from landscaping supply houses is perfectly safe after a rinse. If there's ever a concern, soak it in a bucket and test it.
