Or it can come a meteorite from the sky and wipe the whole tank out How - in the whole world - should gas residues survive a cleaning with water. should you not buy stones from a LFS near a busy street or in the city?. The picture shows a couple of very clean rocks and if you get in some bacteria - it is only good for the diversity of the tanks microbial fauna.OP said it was a busy street. Gas pollution can easily wipe out the tank.
I am not young enough to know everything, but neither so old that I forgotten everything