I beleive it is fairly typical... except for perhaps in more heavily fed tanks at least...
Yeah, I'm wondering about porosity. It's funny that you started this thread because I was basically wondering the same things and the thought had occured to me just today about the rock density.
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-- Tony
My next hobby will be flooding my basement while repeatedly banging my head against a brick wall and tearing up $100 bills. Whee!
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