Plecos are omnivorous but, in the wild, feed mostly on plant material at night. During the day, their unusual omega irises block a lot of the light out of their eyes, but they are usually open at night. They can roll their eye within their sockets.
As they age, their foreheads enlarge in a peculiar manner. Plecos may become more territorial with age and are best kept individually in tanks. Because of their potentially large size and territorial behaviour, it may be advisable to procure a less aggressive catfish. In a suitably large tank, a solitary plecostomus will live amicably enough in a community alongside other tropical fish. These catfish can be kept in tanks with "cold-water" species like goldfish, but will do better if the water is heated.
There are a number of species that are sold on the market under the name common algae sucker, including Hypostomus plecostomus, Hypostomus punctatus, Pterygoplichthys multiradiatus, and Pterygoplichthys pardalis.
There are many types of suckermouth armoured catfishes that science has not described. As a result, they are given a common name and an L-number designation until a new scientific name for the fish is described. An example is the flash plecostomus, L204, believed to be a species of Panaque.
Plecostomus catfish are some of the most commonly kept algae-eating catfish, and are also some of the largest. Individuals measuring over 60 cm (2 ft) long have been reported in ponds and large tanks; most people think they only grow large enough for their aquarium, but a small tank may only slow their rate of growth. Their growth may also become stunted in a smaller tank, leading to bad health and possibly an early death.
Plecos, when introduced to an aquarium, will often find a permanent resting place (under or inside an ornament or rock, for example) to spend most of its time. This becomes a "home" for the Plecostomus. If there are no hiding places, they will sleep on the corner of their tank.