rinse really really well. boil if you can, bake if you can't, and then rinse again. If you have no other acids available, put some vinegar on the rock. If it starts sizzling, then you shouldn't put it into an aquarium. If not, then let it sun dry if possible (again, baking works), and then put it into you aquarium. Keep an eye on your PH and other levels to make sure the rock isn't affecting anything.