Hi. If you donīt want to change the water parameter, you must have carefull with stones. A good way to identify the stones does not contain minerals that can change the parameters, is pour hydrochloric acid on the stone (after washing and dry). If the stone bubbling is bad symptom, is from alkali origin, if it does not, is perfect for use in the aquarium. you can also use vinegar, but with the hydrochloric acid is more visible.
The slate is perfect, but carefull not all stones slabs are slate stone. The silicone must be special for aquariums.
Regards
Loren
Carefull with acid is very dangerous
