nothing, just long time use. key to any scaping is to use what is most pleasing to your eye. slate is good for making structure, and even screwing down pieces of wood that wont stop floating.
stuff with copper sulfate in it will generally kill snails quickly. to be noted, it will kill any snail,shrimp,crab, it doesnt care. also in heavy enough doses that are sometimes required to eliminate snails, it can harm fish.
that stuff is spooky first time you get it. its an ohhh crud whats happening moment. it will go away in time. some pieces of wood take 2 or 3 weeks, some take a couple of months. its ugly but harmless.