Plecos, when introduced to an aquarium, will often find a permanent resting place (under or inside an ornament or rock, for example) to spend most of its time. This becomes a “home” for the Plecostomus. If there are no hiding places, they will sleep in the corner of their tank.
Some fish keepers have trouble housing Plecostomus in heavily planted aquariums because some Plecostomus thrash their tails to develop a “well” in the gravel of the aquarium. It is often necessary to replant aquarium plants that they dislodge.