they certainly poop a lot lol i got a 7" and an 8 " common aswell and my leopard plec (orange) and some bn's in 300 litre tank
the only things that stop the poop on the sand is i have at least 40 (have bred and bred ) corys in the tank with their constant digging around the sand is only dirty for about half an hours then the poop flies into the filters
also have lots of MTS snails constantly turngin over the sand - only way i found to keep the sand clear when keeping plecs in the tank - without having to vac all the time