You put them in a clean tank for 3 days without feeding them so they poop out all there poop.
But you dont need to do that, if you dont they might taste a little bit more muddy or have more poop in there poop pipe. But you take the poop pipe out when you shell them anyway.
My Familly grow them as a Buisness down here

(well until the drought came and we got water restictions and out water pump down at the lake where we got our water from the ponds turned into a land pump)