I'm way over budget on this tank so cheap/found rocks are a must. This was meant to be a cheap joint christmas present for my daughter and myself but I've ended up getting obsessed with aquascaping! I always feel happier looking at tanks where the fish have a natural landscape to live in.
There was a place up my way that sold quarry waste for soakaways and backfill, if I remember right I think it was Basalt. Some very interesting pieces on a par with many "scaping" stones I've seen. The biggest difference being it was £25 per ton
Be careful where your sourcing your hardscape from with commercial locations. You don’t want anything that has been in contact near any fuel, oil, or any other type of contaminant that would decimate your tank.
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.
If you found a rock, that you weren't sure about, all you would need to do is place it in a container with some RO (or rain) water. Wait a couple of minutes, then <"measure the temperature">, measure the conductivity, then wait a couple of days and repeat the process.
You can see for calcium carbonate it says "insoluble", but a lot of water is "hard" (high dGH/dKH) and saturated with Ca++ and HCO3- ions which doesn't seem to make a lot of sense, this happens because calcium and magnesium carbonate are soluble in acids, and rain-water is naturally slightly acid.
If the rock is inert, the conductivity of the will be the same.
If the rock is <"slowly dissolving">, some of the limestone (CaCO3) will have gone into solution as Ca++ and 2HCO3- ions. A greater number of ions conduct more electricity and the (electrical) conductivity of the water will have gone up.
I'll be honest I wouldn't even <"bother testing a rounded cobble">, the fact that it is rounded means that it is incredibly hard rock and hard rock is always impermeable.