On the boards people refer to their sucker-mouth catfishes as "plecos" or plecs (or pl*cos, but I'm not getting into that discussion). They may be writing about anything from an Otocinclus catfish at two inches to an imported Snow King Plec at twenty-four.
These are all members of the South American catfish family Loricariidae. These are mostly herbivores (algae, plant and fruit eaters), but not all- some are micropredators. They share a distinctive sucker mouth, which is well adapted for rasping algae from wood or rocks. Most are rather compressed and flat-bottomed such that they can hang onto plants and rocks against currents. Back in 1991 when Dr. Carl Ferraris, Jr. wrote Catfish in the Aquarium (Tetra Press), he said this was by far the largest family of catfishes, with over 450 species recognized at that time. Many more are listed in some manner now, even if not officially named. AquaLog has published a volume of the magazine with photographs of what were then as yet nameless but "L-numbered" Loricariids in commerce. This reference can be quite helpful for at least tentative ID.
Some of these beasts have been known for a long time. One of the earliest species kept in tanks was originally named Plecostomus plecostomus (Linneaus). Due to revisions by taxonomists (constant in this family), the original genus was found to be invalid, and the fish became Hypostomus plecostomus. Later the species name itself was changed and/or divided, and the name "plecostomus" was no longer more than a synonym for other names. By then however, the original name had become the popular contraction "pleco" to refer to this whole group of armored suckermouth cats, including Hypostomus, Pterygoplichthys, Ancistrus, Panaque, Peckoltia, and others .
Another major grouping of this family would be the whiptailed cats, with Farlowella, Rinelocaria, Sturisoma, and such.
The third major group familiar to aquarists is Otocinclus and relatives.
One problem with these fish is the starvation and stress to which the fish are subject during capture, holding, and shipping. It is understandable for the collectors, shippers and wholesalers to minimize feeding in order to reduce waste and resultant fouling of shipping bags and holding vessels. However, some of these fish are received at the LFS with severely sunken bellies and may be difficult or impossible to restore to health. Most members of the family have somewhat rounded bellies normally (less so in whiptails, or at least it is harder to see). Only consider purchase of fish that are eating and show convex, not concave bellies. Most fish that eat in the LFS will be able to make the transition to your tank without problems. Should the fish have been so starved that eyes appear sunken, it is unfortunately dead but does not realize it yet. These fish do not seem to recover even with the best of care under ideal conditions.
Despite being armored, these fish need some type of refuge for security. For the Oto and Whiptail groups this need is met by plant thickets, but driftwood or bogwood will be used also, especially by the Whiptails. The pleco group seems to prefer wood, particularly tangles of driftwood that they can use for overhangs and caves. Several of them spawn in such sites, particularly the smaller types. Larger plecos tend to spawn in caves they dig in clay stream banks, making tank breeding even more difficult. Most will utilize smooth rock piles and caves for refuge also, and tend to prefer rather close quarters. Beware of undermining of rocks by the fish in efforts to customize their refuge/cave. They are not Cichlids, but can move gravel. When they dig their nose in and flail with their strong muscles, I promise you they can move gravel quite well. Ancistrus and Hypancistrus are particular good at this in my tanks, but at rather limited scale. Any of the pleco group may show the behavior. Whenever possible offer a variety of refuge sites to give them some choice in the selection. Conflicts with other refuge-seeking fish are possible, even common if the tank is a bit more heavily stocked with these fish than it should be.
Almost all the Loricariids need very clean well-aerated water. They like current and dislike water with excessive nitrates. Bulldog or rubbernose plecos seem the most demanding in this regard. Being from the Andean foothill streams, they must have highly oxygenated water to survive. Most (not all) would prefer somewhat soft acid water, but they are an adaptable group for this factor.
Most of the family are strongly nocturnal and may seldom be seen in normal light. The still expensive Zebra Pleco is one of these that prefers a refuge offering complete concealment. Ancistrus and Peckoltia are not far behind, but at least do occasional daylight forays, especially when food is added to the tank. The Royal Clown Panaque always has some disappearing site, but also rests periodically in plain sight. If there are fresh veggies in the tank, he stands guard until it is gone, completely ignoring the light. I have read and heard that they are less photophobic than most of the family, and I do agree with that. They are also the slowest growing Loricariid that I have kept.
A related characteristic of the family is the "Omega Eye". Instead of controlling the pupil of their eye by expanding or contracting the iris as we do, plecos have a peculiar-appearing lobe of the iris extending into the pupil of their eyes. This lobe expands and contracts in response to light levels. leaving a U-shaped, or omega-shaped pupil.
The problem on the boards arises when people use pleco for any Loricariid. Similarities, they have lots. Differences they have as well. Generally they need clean well-aerated water and supplemental food specific to them rather than community leftovers. Generally they are territorial towards conspecifics (conflict over feeding and/or breeding grounds and refuges is obvious) and may be territorially defensive toward other non-related bottom-feeding or dwelling fish. Suitability of housing and as tankmates varies tremendously within the family. I confess I get really uptight when someone lists the inhabitants of their ten-gallon tank and include a sailfin pleco.
So this is really a plea to find out, before purchase, at least which group of the family your prospective fish fits into, and hopefully some idea of how big it would like to be. And if you have a question regarding an Oto-like fish, or a whiptail-type, please specify. Even more, if it really is a pleco, some idea of type is helpful- common pleco, sailfin pleco, small (not young) pleco etc.
Feeding these fish is a subject in itself, so we'll handle that next time.