Oysters are filter feeders that can help clean up the Chesapeake Bay, right? Many have seen the various web videos showing a dozen or so bivalves clearing a murky fish tank in just an hour. But are they such ecological superheroes that each one can siphon 50 gallons of water in a day? Is it true? Well, yes, but not so much in the real world, according to Matthew Gray, a scientist at the University of Maryland Center for Environmental Science’s Horn Point Laboratory.