Actually, the first skeleton building corals were around much longer ago than that. 400 million years ago. The Devonion Age, or the Age of Fish.
I know this because I an sitting on one of the largest Devonion Fossil beds in the World, at The Falls of The Ohio State Park. We have lots of stony coral fossils, many look much like favias, favites, acroporas ,etc.
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GPH tank turn over numbers is about as accurate a method as watts per gallon.
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