Sucker fish or suckermouth catfish are the common names for bottom-feeding fish such as algae eaters or one of the types of plecostomus fish. They spend a lot of time in the aquarium sticking to the walls or the ornaments with their mouths. They are popular as pet fish as they help to keep the tank clean. They will also eat any dead fish.
The misconception is that the sucker fish will get all of the food it needs from the natural algae build up in the tank. This is not true, especially for growing plecostomus fish. There simply will not be enough algae in the tank to keep them well fed. A hungry sucker fish will begin to suck on the skins of any other fish in the tank, which is a danger sign that the fish is starving. Since sucker fish are mostly active at night, it is best to feed them at night right before turning the tank lights out.
Good commercially available food for sucker fish are algae wafers. These sink to the bottom of the tank, where the sucker fish prefer to eat. They will also eat any of the flake or pellet food that has drifted down that the other fish in the tank missed. Sucker fish will not eat at the top of the tank such as goldfish or many other kinds of fish. Sucker fish lack a swim bladder in order to help them stay at the bottom of the tank (or, in the wild, the river bottom).
Sucker fish prefer vegetable matter as treats, but also like sinking shrimp pellets. They also leat canned peas, canned green beans, cooked and cooled zucchini, raw zucchini and cooked and cooled peas. If the treats will not sink, they can be added to a small device called a produce clip. This is a small device that sticks onto the tank sides on one end and clamps onto a bit of vegetable on the other. Sucker fish will also eat any live plants, so if you want plants in the tank, they should be plastic.