I need clear pictures of the fish to see the issue.
What sort of filter is on the tank?
How often and how do you clean the filter?
How often do you do water changes and how much do you change?
Do you gravel clean the substrate when you do a water change?
Do you dechlorinate the new water before adding it to the aquarium?
What is the GH (general hardness), KH (carbonate hardness) and pH of your water supply?
This information can usually be obtained from your water supply company's website (Water Analysis Report) or by telephoning them. If they can't help you, take a glass full of tap water to the local pet shop and get them to test it for you. Write the results down (in numbers) when they do the tests. And ask them what the results are in (eg: ppm, dGH, or something else).
Most rainbowfish have fewer health issues when kept in water with a GH around 200ppm, and a pH above 7.0.
What do you feed the fish?
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It's not hole in the head disease. That is a disease that primarily infects cichlids kept in dirty tanks and is caused by a parasite called Hexamita. Metronidazole kills Hexamita.
A lot of rainbowfish carry Fish Tuberculosis (Fish TB) and this can start out as a white pimple that turns into an ulcer after a few days. The ulcer gets worse and eventually the fish die. There is no treatment for Fish TB. Ulcers can sometimes be treated with medicated food (antibiotics in fish food).
It might not be TB and could be just a sore and malnutrition. Rainbowfish need lots of plant matter in their diet and at least half of their diet should be plant based.
Salt baths don't do anything except stress the fish out. If you want to add salt, add it to the tank for a couple of weeks. If you give a fish a salt bath, you treat the parasites on the fish but don't do anything to the parasites in the main tank. So when you put the fish back in the main tank, it simply gets reinfected. If there are diseases in the main tank, you need to treat the main tank.