Should You Give Your Saltwater Fish a Freshwater Bath?

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Giving your fish a "bath"—or "dip"—may seem counterintuitive, but by switching up salinities on your fish, you can provide a therapeutic benefit. Your veterinarian may prescribe a bath treatment to help your fish through quarantine or to treat a pesky parasite. Please ensure your fish is actually in medical need of a freshwater bath prior to sticking them in just to "see what will happen." Your aquatic veterinarian will be able to diagnose your fish's ailment and determine if a bath is necessary in addition to giving specific instructions.

Why You Should Give Your Saltwater Fish a Freshwater Bath

In marine and freshwater aquariums, your fish may be carrying parasites. You likely do not see any signs of disease because they are at a low level given your fishes' robust immune system. If your fish gets stressed, from poor water quality or inadequate nutrition among many other things, their immune function drops, allowing pathogens, including parasites, to proliferate and terrorize your tank. Sometimes, individual fish just don't have the greatest immune function and will occasionally get sick for no apparent reason.

Bath treatments are often given for parasite infestations. Given their simple nature, by rapidly changing the salinity of the water, your fish's parasites will explode from water entering their bodies and no longer be a pest.

What Happens During a Freshwater Bath

The principle behind a freshwater bath for saltwater fish comes down to simple osmosis. When a fish is moved from saltwater to freshwater, water will move into your fish's tissues to create equilibrium. This happens primarily through your fish's gills and skin. If left in freshwater for more than a short while, your saltwater fish will die because their bodies are not set up to handle that much influx of fresh water.

For hitchhiking parasites, the same process occurs. Since parasites are relatively simple organisms, a quick influx of freshwater causes them to swell and rupture, causing death. Parasites will often die much faster than your fish starts to feel the consequences of hyposalinity. So the goal of the freshwater dip is to eliminate the parasites without hurting the saltwater fish.

How to Perform a Freshwater Bath

Before you begin, you should collect all of your equipment and ensure that it is clean and properly disinfected.


Do not leave your fish alone while they are in their bath treatment. It needs to be monitored at all times while going through this process.

  1. Discuss the proper salinity with your veterinarian. It may not be the best idea to switch right from a salinity of 35 ppt (parts per thousand) to 0 ppt. Depending on the status of the fish, you may want to start with a more gentle decrease in salinity. If you are using a pure fresh water bath (0-5 ppt salinity), be sure the fresh water is dechlorinated and of the same pH and temperature of your saltwater.
  2. If you are using a reduced salinity water (10-20 ppt), weigh out your marine salt mix and carefully measure the volume of your fresh water. You must always weigh your salt. If you use volume measurements (teaspoon, tablespoon, etc.) you are not using a precise calculation. Differences in granularity of salt can severely change your total salinity. Mix the salt to the appropriate salinity by adding the weighed salt into the measured volume of dechlorinated fresh water. 10 ppt is equal to 10 grams of salt in a liter (1000 grams) of water.
  3. Aerate the water using an air stone attached to an aquarium air pump. Double-check the water pH and temperature to be sure they match the salt water aquarium levels.
  4. Get your timer ready and make sure you have nothing on the stove, not running the shower, don't have to leave soon, etc. You must stay with your fish at all times during the bath!
  5. Gently net your fish out of the aquarium with a soft net and place them in the prescribed bath.
  6. Start your timer and watch your fish closely. This is not a very pleasant experience for a fish used to radically different salinity. Usually freshwater dips last up to 5 minutes.
  7. If your fish falls over to one side, immediately stop the treatment and move them back to their main tank! It is normal for them to sit motionless, or swim around rapidly and/or have increased respirations/gill movements, but if they fall to one side, get them out immediately.
  8. Once your timer has finished, gently net your fish and move it back into their main tank. Watch them closely for several minutes to ensure they return to normal activity. If they do not, call your veterinarian as soon as possible.
  9. Throw away your bath treatment. This water should never be retained for future use.
  10. You may need to repeat the treatment either a few times a day or once a day for a few days, depending on your veterinarian's recommendations.

These steps will ensure your fish has a successful treatment with few side effects. If you have any concerns about this treatment, discuss alternative options with your veterinarian. Always make sure your fish has been assessed by a veterinarian prior to proceeding with this treatment to ensure it is in your fish's best interest to undergo this treatment. There is no need for your fish to suffer through an unnecessary treatment because you are unsure why they are sick.