thought about adding some of the beautiful slate chunks and other rocks found in our local creek to a 20 gallon setup; kind of stacking them to make a series of ledges and the like. with all the stuff I am willing to spend money on for this hobby, I just cannot bring myself to pay $2 a pound for rocks at the aquarium store. in addition to the obvious process of removing any aquatic plant or animal growth by vigorously scrubbing, what else should I do to ensure I won't be introducing something awful to my tank? would a good long soak in a vinnegar and water solution do the trick? should I avoid certain types of rock/stone due to mineral content?