i found alot of this rock in my garden which im guessing is slate but not sure and it had patches of black stuff on it which i think is tar i have sterilised the rock/slate and scraped as much of the black stuff of as possible i have put it in my tank but i have no fish in the tank yet and am unsure whether to because i might of missed a bit of the black and i am unsure whether its harmful and what should i do?