Yeah, Pitt is talking about fish and food waste accumulating inside the shells; but don't remove any shell, just clean them once a month turning them upside down while in the tank's water, so that waste come out and then you can remove it. Do not take any shell out of the water to clean it; there may be eggs inside the shell and you should not want to loose them. This is how Pitt told me to do the shells' cleaning and care.