I have been reading a ton about planted tanks recently. I want to set up a nano for a summer hobby, but I am also thinking of starting up my future tank as a planted tank. I have read about using potting soil as a substrate, and I think I'm going to go with that method, but I keep running into an unexplained thing. Each time I read about someone using soil, they also they that they add some crushed coral or crushed oyster shells. I assume this is for the Ph, but I am not sure. Anyone know what this is for, and if it is necessary?