A FW bath can be done in any suitable container to cause osmotic shock. Use 80% FW / 20% aquarium water to help reduce stress. Raise the pH to that of the aquarium by the addtition on sodium bicarbonate. Approx 1 teaspoonfull per gallon, monitor your pH readings closely.

This is assuming this is the treatment of a fish suffering a Parasitism and is being taken from a main display for treatment or during the transfer from a quarantine to the main. Remember before transfer from quarantine, the pH and specific gravity of both tanks need to be adjusted to close paramiters.

IMO a FW dip would be contraindicated whenever a fish has been in a transport bag. I will not bore you with all the details, however, the pH in the bag will have dropped dramatically. This is due excrement and urine in the water. This is why we slow drip acclimate our pets.

Finally; gently ease the animal into the water. Observe resperations. If resp becomes erratic return to aquarium. Fish lying on their sides is normal, observe closely. Never leave the animal unattended and no longer than 10 minutes. Most fishes begin to show signs of discomfort after only three to five minutes.