Eddie,
Its kinda hard to know if it will hurt the fish or not, but I do tend to ask myself, why did the people you got it from pull it out of there tank only after a week or so? Allso that rock came out of the ground somewhere, now does it have any minerals on/or in it that could hurt the tank such as copper? Most likelly no it doesent, but you dont know that. Thats why most people wont use it in a saltwater tank, being a reef or f/o its just not natural to the ocean. But ultimatly its your call, if it was me I would not use it. you might can try it, its up to you.
Good luck