The fish has fungus or excess mucous on its head. What other fish are in the tank?
Livebearers need hard alkaline water with a pH above 7.0 and a GH above 200ppm (250ppm for mollies). What is the pH and general hardness (GH) of the water?
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You can add rock salt (often sold as aquarium salt), sea 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.
If you only have guppies or livebearers in the tank you can double that dose rate, so you would add 2 heaped tablespoons per 20 litres and increase it after 48 hours if there is no improvement 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, gouramis) that are exposed to high levels of salt for an extended period of time, and is not an issue with livebearers.
The salt will not affect the beneficial filter bacteria but the higher dose rate will affect some plants. The lower dose rate will not affect plants.
After you use salt and the fish have recovered, you do a 10% water change each day for a week. Then do a 20% water change each day for a week. Then you can do bigger water changes after that.
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To work out the volume of water in the tank:
measure length x width x height in cm.
divide by 1000.
= volume in litres.
When you measure the height, measure from the top of the substrate to the top of the water level.
There is a calculator/ converter in the "How To Tips" at the top of this page that will let you convert litres to gallons if you need it.
Wipe the inside of the glass down, do a 75% water change and complete gravel clean. And clean the filter if it hasn't been done in the last 2 weeks. Make sure any new water is free of chlorine/ chloramine before it is added to the tank.
Increase surface turbulence/ aeration when using salt or medications because they reduce the dissolved oxygen in the water.
