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Nematodes are relatively large parasites that can occur in the digestive system, internal organs and periodically in the muscle tissue and fins of fish (some species).
There are many species of nematodes and there is no need to focus on identifying individual species here. For the aquarist, it is most important that nematodes do not occur in fish. The only way to tell if discus fish are infected with nematodes is to microscopically examine fresh faeces or organs taken from dead fish.
Below is my older video with a nematode found in discus mucus (magnification x100), posted on YouTube: