Sorry but your fish is too advanced in the stages of dropsy to save but you can try. Salt isnt any use here and will probably quicken the death as the kidneys are probably damaged. Epsom salt is usually whats used at an eighth of a teaspoon per 3 uk gallons but only at the first stages.
As for the main tank I would do several water changes over the next few days and then add some meds for internal bacteria.