Don't confuse salt in your pond as treatment for your fishes problem. Go to the University of Florida's web site that I have a link on my web site to and follow the instructions there.
I use a treatment described there that can only be used for a maximum of 10 minutes or until the fish rolls over. It is agressive and should be done in a bucket with lots of areation. Do not take your eyes off the fish for the 10 minute treatment.
Then, you can use a diluted salt solution, again follow the instructions on their web site.
I would treat them all at the same time, whether they are sick of not, fin rot is contagious.
I use a treatment described there that can only be used for a maximum of 10 minutes or until the fish rolls over. It is agressive and should be done in a bucket with lots of areation. Do not take your eyes off the fish for the 10 minute treatment.
Then, you can use a diluted salt solution, again follow the instructions on their web site.
I would treat them all at the same time, whether they are sick of not, fin rot is contagious.