08-20-2011, 09:22 AM
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In general most rocks are fine. There are some however that may alter water chemistry. A good double check is to pour on a little white vinegar - if there is no fiz, they're okay. If the acid in the vinegar reacts with the rock, it's best not to use the rock in the aquarium.
I'm betting that rocks from a lake, much like rocks from most creeks, streams and rivers are just fine. I have used many rocks from my property lines that are rock walls and piles resulting from cleared lands.
I believe that the rocks need to be processed slightly before adding to an aquarium to better ensure against adding unwanted organisms. There is much [silly] debate about boiling rocks which I don't wish to re-open. You might bring water to a boil in a big pot, then pour over rocks in a 5g pail. Letting sit in the hot water awhile.
Personally, I put mine in the otherwise empty dishwasher with 2+ cups of white vinegar. The DW is set for high temperature and a normal wash. You could adjust parameters and use bleach if you're so inclined.
This will ensure the rocks have been treated such that unwanted 'wild life' will not be introduced to the tank.