Cool thanks
Hendre
Would love to hear if
Stanzzzz7
has any further insight too.
I have yet to find a rock that can't be used in fish tanks.
Some rocks like limestone and other chalky type rocks, as Hendre mentioned can buffer the water. The buffering capacity can vary from one type of rock to another. I have found the buffering capacity of rocks like limestone hardly noticeable, especially when combined under a regime of large scale water changes.
If you wish to keep c/a cichlids or African rift lake cichlids, any naturally sourced rock that is free from contamination should be safe.
If you wish to keep soft water species then do the vinager test to be sure they have no buffering capabilities.
Smooth round rocks found on the beech and round water worn river rocks are normally inert. Chalky rocks like limestone tend to be the ones that harden water.
Simply scrub clean any collected rocks and pour boiling water over them, you should have no issues and it's probably a lot less risky than sourcing your own wood.