Catfish can really add to an aquarium. Some catfish are very interesting alone, and sometimes even comical. There are tons of kinds of Catfish, and they are all special in their own ways. Some catfish, on the other hand, are not acceptable to be kept with Discus. Although most Catfish go well, some don't, i wouldnt reccomend anything that grows to over 7''- 8''. I am saying this because it will add to the stress of the Discus because they aren't the "Kings" and because the catfish may bother them, or just plain be too big for your tank. Catfish are found in the wild schooling with the Discus so why not keep them at home too? Try what you want and see what works JUST BE CAREFUL!! Below are some Catfish that could work in a Discus tank.
Corydoras Catfish --
Corys are very cool fish. They are very lively and come in many colors and patterns. Grows to 2" - 3". Very peaceful and help clean the tank.
Brochis are similar to Corys, as you can tell, but they are less delicate. They can live in hard water, if you have it, which you shouldn't with Discus, but they can live in soft water also. Grow to about 2.5" - 3".
Oto cats are very cool fish. They are similar to Chinese and Siamese Algae eaters, and are known for their triangle shped heads. Like to eat algae all day long. Grows to 2".
Banjo Catfish are really weird looking. They like to hide all day and bury themselves in the gravel. Peaceful, and some are bred to grow to only 4" while others can grow to 11".
Whiptail Catfish --
Cool looking fish. They are related to Banjo Catfish and Plecos. Can stay smaller, but grow to about 6". Watch out for them sucking on the sides of your Discus,
Plecos can help in a Discus tank, but if you can try and get the smaller ones, try to avoid the large ones that grow to 1.5'. Watch out for the plecos sucking on the sides of the Discus. This will cause harm to their skin, and may even kill them.