Ken, I am always using old potting soil mainly because I'm cheap I guess. What I discovered the hard way is that I can use it if it has vermiculite in it but not if it has perlite. I've found my pond too many times with a layer of white floating perlite that worked its way out of the soil. It is a pain to scoop it out with a strainer, but that's the only way I've found to get it out of the pond.
