what type of slate is safe for aquariums without killing fish

[TASHANCRAIG]

i want to build up devided layers in my tank with caves inbetween for zebra plecos but trying to fing ones that are safe as some contain iron and stuff ???? any help would be great expensive delicate fish so gotta be safe

 

[Jason7894561230]

Put some Acid on it (Viniger or Hydrochlocic) and watch for bubbles that will appear if bubbles do appear then the rock is unsafe in the aquarium if none do than its fine.