Have you tested the tank water for ammonia, nitrite, nitrate & pH?
Ammonia and low pH can do that.
There is no red marks/ lines on the tail so it is unlikely to be a bacterial infection.
You should test the water for ammonia, nitrite, nitrate & pH.
Then do a 75% water change and gravel clean the substrate each day for a week. Make sure any new water is free of chlorine/ chloramine before it is added to the tank.
If the tank has been running for more than 2 months, then clean the filter if it hasn't been done in the last 2 weeks. Wash filter materials in a bucket of tank water.
--------------------------
Kanaplex (Kanamycin) is a strong anti-biotic that was designed for people and should only be used as a last resort to treat known bacterial infections that have not responded to normal fish medications. Improper use or mis-use of anti-biotics can lead to drug resistant bacteria that can kill people, animals, birds and fish.
If you use that anti-biotic without checking the water quality, you will be wasting your money.
Anti-biotics like Kanamycin will wipe out filter bacteria and you should monitor the ammonia and nitrite levels during and after use.
Test the water and do water changes for a few days and see if it helps. If it does then return the Kanaplex and keep doing water changes.