Posting some guideline info here from HF that I tend to follow for FW dipping. Can you verify the procedure you did for fw dip.
What "Freshwater" To Use: There are 3 options:
- RO or RODI (add a little saltwater to improve conductivity so that pH can be read accurately)
- Water taken from a "clean" freshwater aquarium (non-acidic i.e. pH should be 7 or greater)
- Dechlorinated tap water (non-acidic i.e. pH should be 7 or greater)
^^ With all of the above, pH should be raised to ~ 8.0 before placing fish in the dip water. This can be accomplished by using supplements (available at most LFS) or you can “bake” your own DIY supplement by using baking soda. Spread baking soda onto a clean baking sheet, and bake at 300F for 1 hour. This process drives off carbon dioxide and water from the baking soda, and the result is an effective pH booster. You will need to experiment (start with a very small amount) to determine how much is needed to raise your pH to the desired level.
How To Treat: Fill a bucket with freshwater, and use a heater to match temperature to the fish's source water. Aerate the water heavily (by using an airstone or small powerhead pointed towards the water surface) for at least 30 minutes prior to doing the dip; and then reduce (or even discontinue) aeration while performing the dip. Place the fish in the freshwater (FW) dip and observe closely. It is not unusual for them to freak out a little at first. Also, certain fish (e.g. tangs) are notorious for “playing dead” during a FW dip. The important thing is to watch the fish's gills (they should be breathing heavily), and keep the fish upright & swimming at all times! If breathing slows, or the fish refuses to get up after chasing (BUT NOT TOUCHING) it with your hand, then it’s time to exit the dip!
Dip the fish for no more than 5 minutes. Multiple dips may be done, but it’s important to give your fish 24-48 hours to recuperate in-between freshwater dips.