My brother is a installs granite for a living, I asked him about the stone recently, and he says almost all granite is treated with a sealer, that I belive would be very toxic to fish. The stone can be stripped of the sealer with acetone, but I'm not sure if you can strip the stone of acetone by soaking it in vinegar. I have been wanting to try it but I don't want to lose any fish if it doesn't work.