I wanted to decorate my tank a bit better and was looking for a way to save money. I look at the LPS and they have rocks that are so expensive although I see the same ones at like garden centers and stuff. Can someone tell me some rocks that I can buy just anywehre that are safe for a tank. Like I know some will deteriate and cause PH changes. Is there something to do to these rocks to make them safe. One that I really like is a flat greyish rock look something like flagstone or something like that???? any one know what I'm talking about???