I may be totally off base, but as a kid I spent many a day in the creeks(or cricks...depending on what part of the world you're from) and rock piles here in TN and I found pieces like that here too. Its fossilized coral. I have no idea its effect upon aquarium water, but your tank looks good. I was big into dinosaurs and fossils as a kid, still love to see the newly discovered bones of the past. Ben...ps-let me know if I'm wrong about the fossil.