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Lol that's almost always my first impulse. Do you use a turkey baster or something to blow off your rocks? If so tap it if it's hard I'd say coral skeleton if soft I'd say sponge
Ya the gha makes me think sponge even more. Don't know if this is the case for everyone, and have never actually tested it but IME it seems sponges pop up more frequently and get bigger in "dirty" water