I heard that only certain kinds of rocks can be used to decorate fishtanks. I have some petosky stones and shiny black rock i think is obsidian (spelling?). i also want to know what kinds i should not use. i also heard i am supposed to boil them first. any info would be appreciated.
Generally what you need to be concerned with using rocks you obtain from "natural" sources is that all biological material and dirt is removed and that they have had any algae, bacteria, fungus, etc destroyed prior to be adding to your tank. Boiling for a long period of time will certainly destroy many of these items. In terms of types of rock you do not wish to use... things like limestone will disolve in water. You should test most rock by splashing a little vinegar on the rock to test if it fizzes. If it fizzes then it will eventually disolve in the water leaving you with pH between 7 and 14 and eventually non-existant or tremendously changed rock.