A fish in distress will itch and rub against the tank. If there is a parasite infection there will be sign of redness and swelling on the body (especially at the bottom of the dorsal fin). You might also notice that the fish has difficulty breathing and is generally inactive. However these are general symptoms of illness, so you need to take mucus and gill samples to detect the parasitic infection.There may be more than one type of parasite, and you need to identify the current stage in the life cycle of the parasite. Hence you need different treatments for an effective cure.Go read about “What types of parasites can attack the fish in your home aquarium?“.