It could be gill flukes or a fungal or bacterial infection in the gills. The fish could also have an internal bacterial infection.
Is the fish eating normally?
What does the fish's poop look like?
If the fish is eating normally, you could try moving it into a spare tank and adding some salt, (see directions below). If you don't have a spare tank you can add salt to the main tank but I would do a big water change and gravel clean the substrate first, and clean the filter too. (Wash filter media in a bucket of tank water and re-use the media. Tip the bucket of dirty water on the lawn/ garden outside). A small water change doesn't do much to clean up the tank conditions. You want to do at least 50 (preferably 75%) water changes each week to dilute nutrients and disease organisms in the water.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
If the fish has stopped eating and does a stringy white poop, it has an internal bacterial infection and will probably die in a day or two. There's not normally any treatment for internal bacterial infections because once the fish shows symptoms, the damage has already been done.
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SALT
You can add rock salt (often sold as aquarium salt) swimming pool salt, or any non iodised salt (sodium chloride) to the aquarium at the dose rate of 1 heaped tablespoon per 20 litres of water. If there is no improvement after 48 hours you can double that dose rate so there is 2 heaped tablespoons of salt per 20 litres.
If you only have livebearers (guppies, platies, swordtails, mollies), goldfish or rainbowfish in the tank you can double that dose rate, so you would add 2 heaped tablespoons per 20 litres and if there is no improvement after 48 hours, then increase it so there is a total of 4 heaped tablespoons of salt per 20 litres.
Keep the salt level like this for at least 2 weeks but no longer than 4 weeks otherwise kidney damage can occur. Kidney damage is more likely to occur in fish from soft water (tetras, Corydoras, angelfish, Bettas & gouramis, loaches) that are exposed to high levels of salt for an extended period of time, and is not an issue with livebearers, rainbowfish or other salt tolerant species.
The salt will not affect the beneficial filter bacteria but the higher dose rate (4 heaped tablespoons per 20 litres) will affect some plants and some snails. The lower dose rate (1-2 heaped tablespoons per 20 litres) will not affect fish, plants, shrimp or snails.
After you use salt and the fish have recovered, you do a 10% water change each day for a week using only fresh water that has been dechlorinated. Then do a 20% water change each day for a week. Then you can do bigger water changes after that. This dilutes the salt out of the tank slowly so it doesn't harm the fish.
If you do water changes while using salt, you need to treat the new water with salt before adding it to the tank. This will keep the salt level stable in the tank and minimise stress on the fish.
When you first add salt, add the salt to a small bucket of tank water and dissolve the salt. Then slowly pour the salt water into the tank near the filter outlet. Add the salt over a couple of minutes.
