Do you know your water out of the tap, pH KH
You can take the rocks, look them over for living creatures, dry them out in the sun for a few days on each side. Then rinse with water conditioner which helps remove heavy metals, like Prime water conditioner. Soak in 7x treatment for volume of water for a day or so. Rinse and dry in the sun again.
Issues with pollution can be a consideration, why the soaking.
These stones look like granite, like the kind used to maintain beach structure more so than coral / reef rock.
They might raise the general hardness.