You'll often see these fishes used as algae eaters.
They will do a fine job of polishing the aquarium glass, but beware
that they will also polish off most any plants! (except for Java Ferns
so far!) Not suited for most planted aquariums, but can be useful and
interesting to watch in an unplanted one...
Click the images to enlarge them:
Plecos need to have some uncoated driftwood in the tank to gnaw on.
Plecos will eat lettuce leaves, and slices of fruits such as papaya, mango, nectarine, and apple. (see above picture)
They can get BIG. A foot long or more. So plan for that when buying one.
Some people keep these quite successfully in goldfish tanks to eat
algae. Keep the temperature 76-78 degrees F. for this compromise
situation, and watch your pleco closely. Depending on individual
temperament, there have been reports of them sucking on the sides of
goldfish. I've never seen one do this myself. Perhaps these are
instances where they were not well fed.
Plecos are a suckermouth catfish. There are bony plates on their sides,
and hard spines in their fins. Sometime they prop themselves up on
their fins.
There are many kinds of suckermouth catfish sold as various "plecos" in
the aquarium trade. Many kinds do not make good algae eaters, but look
cool, so research what you are buying.