You ever walk over to a railroad and look at all those rocks surrounding the tracks with perfect geometric shape? They look something like this: (?)
I would suggest using these for a new aquarium to make caves. As long as they don’t have pesticides, and you aren’t trespassing, you can use these all you like. They’re usually perfect for caves, especially for loach tanks (my loaches love how smooth they are), and can also look natural in the tank itself. I have literally seen a small pond of tadpoles that had these as well as a sand substrate, and they were fine. This is of course a suggestion.
I would suggest using these for a new aquarium to make caves. As long as they don’t have pesticides, and you aren’t trespassing, you can use these all you like. They’re usually perfect for caves, especially for loach tanks (my loaches love how smooth they are), and can also look natural in the tank itself. I have literally seen a small pond of tadpoles that had these as well as a sand substrate, and they were fine. This is of course a suggestion.