How to give a Seahorse a Freshwater Dip

By Dan Underwood


Occasionally a seahorse hobbyist runs into a situation where a freshwater (FW) dip is indicated. A FW Dip can be used as both a therapeutic and a diagnostic tool. As a therapeutic tool it can help rid the seahorse of ectoparasites on the body, in the oral cavity, as well as in the gills.   As a diagnostic tool, observation during the dip will give you a good idea if there is a parasitic load or not. It can also be done prophylactically on new arrivals from suspect sources, on wild caught (WC) specimens or when a tank mate has had known parasitic load.

We have been doing FW Dips for over 11 years.   We have found that every species we have encountered has handled FW Dips just fine. This includes H. abdominalis, H. barbouri, H. comes, H. erectus, H. fisheri, H. kuda (both hilonis and taeniopterus varieties), H. procerus, H. reidi , H. whitei and H. zosterae. We have never lost a seahorse to a FW Dip.

The reason we like FW Dips is that they are easy to do, quick, noninvasive procedure that doesn’t require medications and often produce quick results. Also, during the procedure, one can get a very good idea if there is a parasitic load or not.

How it works

Seahorses are complex, multi cell organisms whereas many parasites, such as protozoans, are simple single cell organisms. The difference in osmolarity between the saltwater (SW) and FW is the magic key to the dip. The hypotonic environment of the FW creates an osmotic imbalance to the protozoan. This causes water to move into the protozoan to balance it which then causes them to rupture or burst. Some larger more complex parasites in the mesozoan or metazoan class may not burst as quickly as the protozoans, but the change in osmostic pressure shocks them causing them to detach and in many cases, perish. Seahorses being a much larger more complex organism can handle the osmotic change for a much longer time and are able to compensate for the changes during the time it takes for a FW Dip.

When is a FW Dip Indicated

There are several instances where one may want to consider a FW Dip.

On new arrivals that are from suspect sources or with WC specimens

When seahorses are scratching

On specimens that have rapid respirations when there is no other obvious cause

As an initial treatment for Weak Snick

When one sees little organisms on the seahorse

As a prophylactic measure when a tank mate has a known parasitic load

When a FW Dip shouldn’t be done

When a seahorse has an open wound or has an obvious bacterial infection.

Materials needed

Container. A clean container to hold the FW and Seahorse. This can be anything from a small tank such as a 2.5 or 5 gallon tank, a large jar, or a bucket. The container does need to be clean and needs to be able to hold enough water to cover the Seahorse in an upright position. My personal preference is to use a 2 gallon glass cookie jar from Walmart. This holds enough water and after the dip I can swirl the water around and concentrate the dropping to examine under a microscope. It is also very easy to sterilize post dip.

Fresh Water. We prefer to use Reverse Osmosis (R/O) or Distilled Water. Tap water can sometimes be used but it needs to dechlorinated.

FW pH Test Kit.

Buffer. This is used to adjust the pH of the water to match the tank.

Thermometer. We prefer infrared thermometers that don’t require touching the water or needing to be sterilized between tanks. This is used to verify the FW and the tank water are at the same temperature.

Airline & Air Pump. The FW will need to be aerated and you should have a bubbler running during the dip to keep the oxygen saturation up.

Hitching post. Some consider this optional. This can be very simple. Just something for the Seahorse to hitch to should it desire to. In many cases they won’t use it but some do.

Timer, clock or stopwatch for timing the procedure.

Mentor or other experienced person. If you are really worried about the procedure or if this is the first time, nice to have someone handy, even it is over the phone to give encouragement, support and answer any questions.


Dissecting Microscope or other magnifier to examine what comes off the Seahorse.

Preparing for the Procedure

The first step is to pick out and setup a container for the FW Dip. Any container that holds enough water to cover the seahorse can be deployed. It should a clean container. Our favorite container for this purpose is a large Cookie Jar from Walmart. They have them in 1 and 2 gallon sizes.   The lid is not used. We like this container because it is cheap, round, being glass very easy to clean, doesn’t have any seems and we can easily view the animal during the procedure.   A bucket, 2.5 or 5 gallon tank can be used as well.

Consider placement of the container for observation and proximity to the display tank. You want the container somewhat close to the display tank so you don’t have to carry the seahorse very far to it and can easily and quickly return the seahorse back to the display tank or hospital/quarantine tank.

Next you will need to add freshwater. R/O or distill water is ideal. Tap water can be used if it doesn’t have any chlorine or chloramines. If either of these are present in your water, it needs to be neutralized prior to the FW Dip.

The temperature of the water needs to be same as the tank or no more than 1 degree different. It can be adjusted prior to adding to the vessel or afterwards depending upon your method of adjusting the temperature. If you have clean shipping bags, the water can be placed in the bag and floated in the aquarium until they are the same. A heater can be used temporarily to raise the temperature. Ice in a ziplock bag can be used to lower the temperature.

An airline will need to be added to the FW. It should be run for a few minutes prior to adding the seahorse, to bring up the oxygen (O2) saturation. R/O, distilled water in a container and tap water often have a very low 02 saturation level. We prefer to use a rigid airline without an air stone but an air stone may be used if desired.

You will also need to test the pH of the display tank where the seahorse is and the FW. If the pH is different (the FW is almost always lower than the tank), you will need to add buffer.

A hitching post may be added to the container. This can be anything that is not too large. It should be clean and either easily sterilized or disposable. A plastic plant, artificial coral or any object that is clean and easy for the seahorse to hitch to can be used.

Performing the procedure

Gently place the seahorse in the FW container. Begin the timing of the procedure and observing the seahorse. You will want the FW Dip to last a minimum of 8 minutes and if there is a reaction to the dip, then you should continue for at least 12 minutes or up to 15 minutes.

The seahorse is likely to react in one of 3 ways after placement in the FW Dip. First it could move about normally or hitch. Second it could play possum and lay on the bottom. Lastly, it could begin to thrash about. Watch for what happens. It is also not uncommon to see what appears to be particles in the water shooting away from the seahorse. This would be little organisms reacting to the FW Dip.

If the seahorse acts normal throughout the first 8 minutes and doesn’t do any thrashing or show other signs of irritability, the seahorse probably doesn’t have much of a parasitic load. In this case, I discontinue the FW Dip at this point and move the seahorse back to the Display tank or Hospital/Quarantine tank.

If the seahorse should just lay on the bottom, look to make sure the seahorse is continuing to breathe. This is a defensive move to make other’s think it is dead. In some cases it may stay there or may get up and move about. Often, if you give it a nudge it will get up. Either way watch for signs of thrashing or irritability.

In some cases, the seahorse will begin to thrash about or become highly irritable. This may happen quickly or can take a few minutes to develop. Their reaction may range from very mild to very active. It can be very disturbing to one not familiar with this process. This is a sign that the FW Dip is doing what it is supposed to be doing. The natural instinct is to remove the seahorse but the best thing to do is to go the full 12 to 15 minutes. What is happening is that the little organisms (parasites) are trying to burrow deeper to get away from the FW and the osmotic shock. This is quite uncomfortable and irritating to the seahorse.   Once you have gone the full 12 to 15 minutes, remove the seahorse and place back in the Display tank or Hospital/Quarantine tank.

If the original symptom or complaint was rapid breathing or weak snick, you may have to wait up to 24 hours to re-evaluate. This is to allow for healing and a reduction of edema within the tissues from the parasites. Some cases may warrant further treatment such as a Formalin Dip or long term Formalin Bath. We don’t recommend multiple FW Dips done in succession. We have seen fungal issues appear when too many FW Dips are done to close together. Generally it is better to consider another form of treatment.

Another consideration when you have positive results of parasites from the FW Dip is to go ahead and treat any other seahorses that were in the same tank. Odds are if they aren’t showing any symptoms, they may soon start to. The procedure remains the same for the other seahorses.

After the FW Dip is done, many like to examine what came off the seahorse. If you used a round container or if you pour the FW into a clean round container such as a bucket, you swirl the water around so that it begins to spin. The contents in the water becomes concentrated in the center of the spin. A sampling can be removed with an eye dropper, small syringe or even a turkey baster. You can then look at the contents under a microscope or other strong magnification. You will likely be surprised at how much stuff you find. It will be hard to identify everything due to the bursting from osmotic pressure, but you will likely find it very interesting.



Further Reading:

Fish Diseases and Disorders, Volume 1: Protozoan and Metazoan Infections, 2nd Edition. Edited by P.T.K. Woo, University of Guelph, Canada 2006

Marine Ornamental Species, Collection, Culture & Conservation,   Edited by James C. Cato and Christopher L. Brown, Iowa State Press 2003

Fish Diseases, Diagnosis and Treatment, Second Edition, Edward J. Noga, Wiley-Blackwell, 2 edition, 2010

Fish Parasites, Pathobiology and Protection, Patrick T. K. Woo & Kurt Buchmann, CAB International, 2012

Rev 10/2016