These are exotics from Africa, Asia and South America and are called Plecostomas (Pleco for short). There are several varieties that now live in our waters. They start in aquaria about an inch long or so. After two years, they are the better part of a foot and those people who can't do the right thing (kill them), release them into the wild where there is no telling what harm they will do to the environment. However, these are not the infamous "walking catfish" which are exotics introduced from South America.
Most can be caught by hand, and I catch at least one for my students to inspect. They do eat a boat load of algae. Do a search for "Pleco" if you want more info.