The common type fish store plecos are nocturnal, and they have been known to actually suck the slime coat off other fish who prefer to rest at night. Fish stores don't always tell you that. Like most people when I started in the aquarium hobby, I thought plecos were fascinating. However, after keeping them for several years, I discovered that they make a bigger mess than they clean up. The last one I had met his demise when the front glass cracked in a 55 gallon aquarium, which subsequently lost all of its water.