How long have you had the fish for?
Have you added anything to the tank in the 2 weeks before this started?
How long has the tank been set up for?
How often do you do water changes and how much water do you change?
Do you gravel clean the substrate when you do a water change?
Do you dechlorinate the tap water before adding it to the tank?
What sort of filter do you have?
How often and how do you clean the filter?
What are the ammonia, nitrite and nitrate levels in the water?
What is the GH (general hardness) and pH of your water supply. This information can usually be obtained from your water supply company's website 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).
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You shouldn't add medications unless you know what the problem is.
Don't use anti-biotics unless the fish have a confirmed bacterial infection that does not respond to normal fish medications. Improper use or misuse of Anti-biotics can lead to drug resistant bacteria that can kill people, animals, birds and fish.
Most fish health issues are caused by poor water quality. The following link has information about what to do if your fish gets sick. It's long and boring but worth knowing.
https://www.fishforums.net/threads/what-to-do-if-your-fish-gets-sick.450268/