Don't use Furan 2 again. If it didn't work the first time, it isn't going to work now.
It is either a fungal or bacterial infection. Fungus is normally white and fluffy but there are some types of fungus that just live in the skin and look like the skin.
Bacterial infections are normally red but a slow growing bacteria will erode away the skin and surrounding tissue.
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How long have you had the fish for?
How often do you do water changes and how much do you change?
Do you gravel clean the tank when you do a water change?
Do you dechlorinate the new water before adding it to the tank?
What sort of filter do you have?
How often and how do you clean the filter?
What is the ammonia level?
Have you added anything to the tank in the 2 weeks before this started?
How much salt have you been adding?
What is the fungal treatment you are using?
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Try doing a 75% water change and gravel cleaning the tank every day for 2 weeks. Wipe the inside of the tank down before doing this.
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. Wash filter media/ materials in a bucket of tank water and re-use them.
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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 2 heaped tablespoon per 20 litres of water.
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 20% water change each day for a week using only fresh water that has been dechlorinated. 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.
If there is no improvement after 2 weeks of salt, then stop using it.
