A "Salt Bath" or "Salt Dip" has many advantages, but it must be done under the correct circumstances. These are as follows:
1. The fish has a lot of parasites
2. Bringing your fish inside from the pond to a tank
3. A new fish, as part of a QT process
4. Perisitant case of fin rot
5. Flashing
You can use aquarium, rock, pickling, solar, or Kosher salt. Do not use table salt as it has anti-caking agents that will kill your fish!
Process:
Use a 1/2 cup of salt per gallon of treated water. Dissolve the salt and make sure the temperature is as close as you can get to that of your tank before dipping the fish.
Start timing the second you put the fish in the water. You can dip between 30 seconds and 5 mins. If the fish shows ANY signs of stress, take it out immediately!
The fish will float to the top (this is normal), but should be swimming around. If it stops swimming, give the fish a little nudge, if it starts swimming again, continue the bath, if the fish does NOT start swimming again, remove it!
Smaller fish need less time in the dip than bigger fish. After all, they are more delicate. If the fish starts to act funny in any way after you put it in the dip, remove it immediately! Regardless of if the fish is acting fine, you never want to dip more than 5 minutes total!
After the dip, you may want to put your fish in a temporary home if the tank is infected with parasites. It will need to be treated, as well.