Plecos, Loricariids, are not coprophagous scavengers. That is a fancy way of saying that they do not eat or clean up fish poop. Rather the opposite, they are prolific poop generators. If a scavenger of fish poop is what you want, a Scat is the only one I know. But they get rather large and like brackish conditions.
Plecos also will not do well with only the leftovers from the other fish. Hopefully there will not be enough of that to keep them going, and it is not likely to be an adequate diet. These fish are "designed" with a rasping mouth to enable them to scrape algae from surfaces, or the pulp of fruit, or even wood itself- not just algae from the wood, but to eat and get nourishment from the wood.
Much of their food is relatively high bulk/low food value. They are "engineered" to process a larger quantity of material than many other fish in order to get equal food value. Obviously this ability to exploit a low-nutrition food source has paid off for the family, as they are widespread through South America east of the Andes, with a remarkable number of species. Some of these species have reduced the family's characteristic dependence on plant-based materials in order to fill niches lacking such resources. At least one population of the currently popular and expensive Zebra Pleco is such a fish. It appears to be more adapted to a micropredator lifestyle than to being herbivorous. But the majority are herbivores. Feeding in tanks is not difficult, but they do need to have a higher bulk diet than most other fish. That point we'll discus in more detail later in this article.
Most of us purchase our first pleco cat for algae cleanup. Most of these fish will do that, with some reservations. The Hypostomus-type plecos will usually do a pretty good job on the tank glass, river rock, and large pieces of driftwood. But they quickly grow too large and heavy to work on plant leaves. They are too heavy-bodied and have insufficient swim bladder function to hover in the water while rasping surfaces. Instead they hang onto a surface while rasping away- fascinating to watch when they are belly-out on the front glass. Once the fish have reached only a fraction of adult size there is no way a plant leaf can support them easily, if at all. So they start ignoring the plants. Or worse, they weigh down the lower leaves of swordplants or such, and rasp away the whole thing. They are not being malicious. They are hungry. They would prefer softer, more readily available food, but if there is not any, they will eat what they can get. This can include just about any plant we grow, with the probable exception of Java Fern. This behavior is not a fish problem; it is a fishkeeper problem. We are not feeding the fish properly. I would also question the idea of keeping any of the large Plecos in a standard planted tank. They are just too big for "Dutch Style" tanks. The bull-in-a-china-shop concept fits here. Actually I believe that any catfish with an adult size in excess of 6" probably should not be in such tanks. Peckoltia and Ancistrus species are about as big as you should go. Probably the best in the family for heavily planted tanks is from a different group, the Otocinclus species. They stay small and agile and do not damage plants. Along with their smaller size, they seem not to be territorial as are their larger cousins. In fact, they need the company of conspecifics for comfort and security.
Algae is definitely a low food value menu item. Do not expect anything but the largest tanks to produce enough algae to support even a medium pleco.
These are the standards for feeding most aquarium fish. In my not-so-humble opinion they are helpful as supplements, but insufficient as basic diet for pleco catfish, especially large ones. The problem is not that they are inadequate nutritionally, but the opposite in a sense. They are too high in protein and much too low in bulk. Read the nutritional information on the container. Protein levels tend to be pretty high. This is great for most fish and as supplement for Loricariids, but protein levels tend to be about 30% or even higher. Consider that in the wild these fish never see anything with that content, unless they are eating a dead fish - which we try to avoid in our tanks. They are programmed to high bulk foods and do not "know" the tablets and wafers we feed them are concentrated. We could say they still "feel" hunger, not because their tablets and wafers are inadequate food value, but because they are inadequate bulk. Feeding more prepared food is not the answer. That route can lead to liver and kidney problems, waste money, and increase the tank bioload needlessly. So please, consider these items supplement, not staple.
This is really the staple diet, after algae, for these fish in captivity. There are many veggies easily prepared for catfish feeding.
Zucchini is the classic fresh veggie. For smaller specimens or more frequent feeding of larger fish, the zucchini can be sliced (after washing/rinsing thoroughly just as you would for yourself) as cross-sections, or for slightly larger slices, diagonally. This slice can be fed directly without other preparation. However, the fresh material floats, so is best used in either a veggie clip (a plastic clip with a suction cup attached, available from LFSs) or by attachment to a rock by a rubber band. The rest of the slices I spread on a waxed paper covered cookie sheet and freeze overnight, then store in ziplock plastic bags frozen for future use. For large specimens and especially Panaque or Hypostomus, I slice the zucchini lengthwise into halves, then quarters, and cut those into 2-3" sections, or whatever size is appropriate for your fish. Excess is frozen and stored in the same manner as the round slices. I thaw these at room temperature and rubber band to a rock. Most of the plecos will not eat the rind, so it should be removed after about 24 hours. I have a couple of Panaque who eat the rind first, so that in not a universal rule.
Cucumbers are another fresh food for the catfish. Many folk hold that the cukes are without food value and should not be used. If you go into one of the nutrition sites and compare zucchini and cucumber, you will find the latter to be somewhat less nutritious than the former, but the differences are not great. As I am a strong proponent of feeding a variety of foods, I see no problem and potential benefit from including this in my fish's diets. Handling of cukes is identical to that for zucchini, but I never slice it thinly as it does break up more easily than zucchini.
Leaf lettuces are also popular and good foods for these fish. Romaine is the most commonly suggested, but most dark leaf lettuces can be used. Iceberg is low in food value and would provide essentially bulk only, I do not use it. Leaves are best either blanched or frozen. Blanching (for those who do not cook) involves placing the leaf in a colander and pouring boiling water over it. You are not cooking it; you are wilting it, which supposedly makes it more accessible to the fish. Freezing has the same effect, as does very brief microwaving. Again clip to the tank glass or use the rock and rubber band. Spinach is somewhat controversial- some folk swear it is toxic, others swear by it. I do use spinach occasionally when it is the stealable salad green available, handled in the same manner as the others.
Green peas and Lima beans are also used by many fishkeepers. The capsule/shell on the individual pea or bean should be removed. This is most easily done by microwaving the peas or beans briefly in a little water, then shelling them. To me this is a hassle; to you it may be easier than slicing zucchini. If it is no problem, these items are something else that can be used in the fish veggie rotation. I confess that I do this only for delicate catfish babies. It was the only way I was able to feed Sturisoma fry, and even that was not very successful.
These foods all provide the bulk that is natural to the pleco catfish. I strongly recommend that you use as many of these as is possible or convenient for you. It does take most fish a feeding or two or three to discover these foods and realize they are food, but after that there is no hesitation. Several of my fish stand guard over the veggies to keep their tankmates away. By the way, you may well find that quite a number of tank fish (other than plecos) will relish these items. My mbuna really get excited with these foods, and a to me surprising number of Tetras chow down as well.
Panaque have been shown to house gut bacteria comparable to those found in termites. The fish rasps away fine wood fibers and eats them. The gut bacteria digest the cellulose from the wood and make the breakdown products of this material available to the fish, so for these at least, wood is a dietary need. All the pleco group seem most comfortable (IMHO) with wood in their tanks, and all the larger ones I have kept rasp it away very slowly, so I do provide wood for the entire family, for some primarily as refuge, but as potential food, exercise, etc. for all.
Some plecos lean toward eating meatier foods, and all of them will enjoy occasional meals of small worms, insect larvae, crustaceans or such. These high-protein treats are no doubt similar to the small creatures they could capture while grazing on algae beds in the wild. I do give frozen bloodworms to my plecos with some regularity, and during gardening season, treats of small (or chopped- don't gag please, they really are frozen and broken) earthworms. The only trick is to feed the cats in mixed tanks without the other fish stealing the treats. I do this using red lights after the regular tank lights and room lights are out, but then everybody already knows I am a bit crazy.
A recurring theme on the boards are pleas for help, shock, and dismay from tank keepers whose pleco catfish are attacking their prize Discus, Gouramis, Silver Dollars, Goldfish, or any other fish with a large enough body for the pleco to grasp with their mouth. They are sucking the slime and sometimes even the flesh from the fish. Obviously, this is a very bad situation and an absolute no-no which could result in the death of the target fish. Sometimes it is a report that plants are being damaged, rasped and scarred. But this is again a fishkeeper problem, not a fish problem. The majority of the attack fish - and it can be any of the plecos - are fed nothing at all. "They are scavengers, aren't they?" "There is plenty of poop and other waste in the tank." "There is algae all over my wisteria, why doesn't the 8" (or larger) sailfin plec eat it?" A small minority is fed sinking wafers. I have never been able to confirm an instance of this extreme starvation behavior where the fish had regular access to fresh veggies. The sad part is that once this behavior is established, it seems to be hard to overcome. I suppose neurotic or psychotic fish are as conceivable as neurotic humans.
It is a good idea to get some idea before any purchase of the nature of the beast that catches your eye. Small sailfins are charming, but suitable only for large tanks- will you be able to trade the fish in when it outgrows your tanks? Not just store policies, but the psychological stress on you, to part with a "friend"? Trying to find exact ID is difficult, many of the fish have not been classified systematically. But to get some idea of at least the genus or subgroup to which it belongs is helpful.
As for selecting individuals, reread the first part of the article on starved plecos in local stores and be conscious of the belly and eyes of the fish you want.
To me, this is a very large and endlessly fascinating family of fish. They are quite adaptable to water conditions, and given a suitably sized and decorated tank, clean water, good oxygenation, and an adequate diet they are generally long-lived. Most are nocturnal, but many will be frequently visible during lighted cycles if there is plenty of refuge and shade available.
If fishkeepers would give up thinking "scavenger" and think instead of herbivorous but algae-preferring fish, their lives would be better- both the fish and the keepers.