i was out looking for decorations for my new tank and found the price of rocks at the lfs simply outrageous. my question is can i take some rock from outside and use them. i know to boil or heat them in the oven to kill anything that might be living on them but what about mineral leeching from the rock itself. the region i live in is part of the cambrian shield and the rock outcropings here are among the oldest on the planet(about 4 millon yrs) would these pose any danger to my oscars??

Your right the price is outragous at some shops, the best places to get rocks is from the beach as they will have no effect on the chemical makeup of the water. The only rocks that will have an effect on your water are those that are soluble (chalk like rock)and they will tend to alter the ph in your tank upwards. The softer the rock the more effect it will have on your water.Rocks found form around the seaside are much harder and are fine for your aquarium. The rock you have in your area should be fine if its been around for millions of years its certainly not going to wear away in your tank. Another place where you might find rocks are in garden centres but they are also expensive but not as expensive as your local fish shop. Try also your local free cycle centre on the internet. This is large groups of people who have things that they want to get rid of but don't really want to take them to the tip. Ive had loads of fish tanks and equipment for free.

ive used large rocks from the beach,gave them a good boil and they never affected my water quality