there is a store that has all kinds of rocks just south of pico on Sepulveda blvd. that sells all kinds of rocks for garden and landscaping. If you go with granite boulders or large river rock or any inert rock will be fine for the tank.
I've bought a lot of cool rocks at landscape supply places(including river rock).Just look up landscape supplies in your yellow pages for the one closest to you. Will probably run you between .10 - .25 per lb. Mike