If the fish has small white spots on the body, then it has white spot (Ich/ Ick). This is a protozoan parasite that affects the gills and covers the fish in small white spots. They eventually kill the fish if left untreated.
You can treat white spot by raising the water temperature to 30C (86F) and keeping it there for 2 weeks, or at least 1 week after all the spots have disappeared.
Before you raise the water temperature, do a 80-90% water change and gravel clean the substrate. This will dilute the number of disease organisms in the water and buy the fish some time.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
Clean the filter if it hasn't been done in the last 2 weeks. However, if the filter is less than 6 weeks old, do not clean it. Wash the filter materials/ media in a bucket of tank water and re-use them. Tip the bucket of dirty water on the garden/ lawn. Cleaning the filter means less gunk and cleaner water with fewer pathogens.
Increase aeration/ surface turbulence to maximise the oxygen level in the water.
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If the fish doesn't have white spots, then do a 75% water change and gravel clean the substrate. Clean the filter, increase aeration and add some salt.
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SALT
You can add rock salt (often sold as aquarium salt) or swimming pool salt 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.
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, 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.
