Red shale rock

[cobra1264]

I purchased serval large chunks of red shale for my fire belly toad tank to make a natural looking water fall. Only issue I have is how do I go about making it safe to put in the mostly aquatic setup to help quiet the water falling out of the filter? I also want it in their to give them a better climb up spot to help shed skin and maintain a realistic look to a river bank the water is almost 5 inches deep and they love to soak but I need to have more land for the 4 of them.