Paul, come round to my place and pick up some guppies.
Some of my young ones to act as sacrificies.
No cost for these, just want to see you get this straightened out.
Make sure that you have scrubbed down the tank with salt and water, so as the salt is still there, like a paste.
Try them in the bare tank first. If they survive the bare tank for a couple of days, then try boiling the stones for twenty minutes and once cool, try them with the guppies.
See if anything happens to the gups.