Fish are susceptible to internal worms and parasites. An obvious sign of worms is when fish become very thin but still continue to feed. When your fish become lethargic, fins are clamped, thin-looking, gasping and showing signs of rubbing/irritation these are clear signs that your fish has an internal disease that requires treatment.
External parasites often cause symptoms like Slime & Velvet. The symptoms include a peppering of tiny gold specs across the body and a coating of excess mucus - slime - on the skin. Slime and velvet are caused by a variety of parasites which will be naturally present in small numbers on aquarium fish.
In a healthy environment, the fish’s immune system will cope with these parasites however fish stress (mostly caused by bad water quality) can lead to major outbreaks of parasites.