How do mermaids poop?!

Last night, as I was lying in bed, trying to get to sleep, a question popped into my mind: How do mermaids poop?! Obviously, they don't do it the normal way, the way you and I do it, so how is it done? They have to poop, because they eat, and they can't store it. They would become really fat, and not have the strength in their tails to swim for food. Plus, no one has ever seen a fat mermaid. I wonder if it goes through their mouth, like how owls poop, through their beak. That would be a little gross though!