Hi all,
I'd go further than that, round cobbles are always safe to use, and any <"
effect on water chemistry will be"> , at most, negligible. Only really hard rocks form cobbles.
Yes, they just need a soak. It is for the same reason, if they were permeable they wouldn't have formed cobbles so any salt etc will just be a surface layer.
In terms of rocks, if you have hard tank water you can use any rocks you like the look of. Limestone rocks won't <"
make your water any harder">, the water is already saturated with Ca++ and HCO3- ions.
cheers Darrel