Hi and welcome to the forum 
It's a bit hard to tell what is on the fish but I am going with a bacterial infection.
When you photograph fish it's best to photograph the fish from the side. Viewing fish from above makes it hard to see what is going on.
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How long the tank has been set up for?
You should check the water quality for ammonia, nitrite, nitrate, pH and general hardness (GH).
What other fish are in the tank?
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You should use a broad spectrum fish medication that treats, bacteria, fungus and protozoan infections.
If you have catfish, eels, loaches or other scaleless fish in the tank, use a medication suitable for them. If you have scaleless fish but can't find a medication for scaleless fish, then use a normal medication at half strength.
If you don't have any scaleless fish then use the full recommended dose.
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Because of the time of year, pet shops might not be open. In this case you can do a 75% water change and gravel clean the substrate and then add salt.
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 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, 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 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.
----------------------
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.
Remove carbon from the filter before treating or it will absorb the medication and stop it working.
Wipe the inside of the glass down, do a 75% water change and complete gravel clean. And clean the filter before treating. Make sure any new water is free of chlorine/ chloramine before it is added to the tank.
Increase surface turbulence/ aeration when using medications because they reduce the dissolved oxygen in the water.
It's a bit hard to tell what is on the fish but I am going with a bacterial infection.
When you photograph fish it's best to photograph the fish from the side. Viewing fish from above makes it hard to see what is going on.
----------------------
How long the tank has been set up for?
You should check the water quality for ammonia, nitrite, nitrate, pH and general hardness (GH).
What other fish are in the tank?
----------------------
You should use a broad spectrum fish medication that treats, bacteria, fungus and protozoan infections.
If you have catfish, eels, loaches or other scaleless fish in the tank, use a medication suitable for them. If you have scaleless fish but can't find a medication for scaleless fish, then use a normal medication at half strength.
If you don't have any scaleless fish then use the full recommended dose.
----------------------
Because of the time of year, pet shops might not be open. In this case you can do a 75% water change and gravel clean the substrate and then add salt.
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 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, 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 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.
----------------------
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.
Remove carbon from the filter before treating or it will absorb the medication and stop it working.
Wipe the inside of the glass down, do a 75% water change and complete gravel clean. And clean the filter before treating. Make sure any new water is free of chlorine/ chloramine before it is added to the tank.
Increase surface turbulence/ aeration when using medications because they reduce the dissolved oxygen in the water.
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