Hey there.
What i did to save my fish
cero chlorine
salt
high water temp
and then full dose of tetracicline <--that's the life saver when the fish is too sick to wait for the salt and clean water to work, by looking at your fish pic maybe you dont need to use this. your water will get red dont panic ,water changes will make it go away
my fish got a bad case of fin rot and gill disease due to frecuent water changes with tap water and then using declorinator
Now i use chlorine filters and a put the water to rest for a couple of days on a diff container before adding it to the tank, and i always test the water for chlorine before using it
Chlorine = Fish killer