This article is from the Aquaria: Disease, Algae and Snails FAQ, by Elaine Thompson, Thomas Sasala and George Booth
Otocinclus are diligent algae eaters, but are best kept in schools due
to their small size. One per 10 gallons is a useful rule of thumb.
Various species of otos are seen in the shops at various times; most
are good algae eaters but some seem to prefer the slime coat on fish
to algae. Unfortunately, there seems to be no way to distinguish the
"attack otos" from normal otos.
Otos seem to be very delicate fish, but this is probably due to
capture and shipping abuse rather than an inherent weakness. When a
fish shop gets some in, it is wise to wait a while before purchasing
to account for die offs. Most people report getting a dozen and having
them die over a period of a few months until just a couple are left.
Those then seem to last for a long time.
Plecostomus is the generic name for a wide range of sucker-mouth fish.
Only the smaller types are useful in a planted tank, since the larger
varieties tend to eat the plant right along with the algae. Two common
types that are useful are the "bristle-nose plecostomus" and the
"clown plecostomus" or Pekoltia. Both stay under 4" long and don't
seem to cause too much plant damage. Sometimes broad-leafed plants
like Amazon swords will be scraped a little too closely by the plecos,
so they bear watching.
Their diet can be supplemented by blanched zucchini and bottom feeder
tablets. They also appreciate a chunk of driftwood in the aquarium to
satisfy their need for cellulose. See the GOOD FIRST FISH FAQ for more
information on keeping suckermouth catfish.