So I'm in process of choosing my materials for creating my paludarium in a 55 for about 4 large, 2 medium leopard frogs. I'm trying to figure out what rocks to use and I know the rules for using them for fish tank but what about frogs? Say if I use limestone or marble, would the water hardness change be too great for frogs or native North American fish like dace? So far I have a list of slate, granite, quartz, and lava rock (not really liking it though) as my options. Any other rocks that would be safe for both the fish and the frogs?