That doesn't look like fin rot...looks like a lot of broken blood vessels. I've seen that happen when the water is foul or way too acidic.
If you left the filter off for a day, a lot of the bacteria in there probably died so put in some live bacteria so your tank can cycle again.
If the bichir is upside down or on it's side, it is probably dust.
Salt is not going to help you, but another huge water change might.