How can I test whether a rock will be toxic in the aquarium? Will the water become harder? Measured Ph and hardness of water and then placed rock in the water and will test in a week. Ph is 6.4 and 2 degrees of hardness. Not bad for out of the tap if you keep dwarf cichlids! The rock is said to be granite and has fleck of a metallic flecks. I have read that some rocks with these flecks will give off toxic substances. Any ideas? Thanks for any assistance.