Depends on the species of pleco and the type of wood. I had 3 ancistrus species, and one medium sized common. All would munch on the wood in the tank. Mopani is denser, and I could see the rasping marks, but never had big chunks missing or fall off. The softer driftwoods were a different story--not only was the rasping visible, but they'd chew enough that I had wood 'litter' all over the tank--little bits that they'd eaten through a branchy area. If I left them in the tank, they disappeared within a few months, so I suspect those little 'splinters' were eaten as well.