Hyposalinity is a very effective treatment for parasitic conditions such as Marine "Ich" (Cryptocaryon irritans.) and can also be used to lower stress in fish.

What is Hyposalinity?

Hyposalinity is adjusting the salinity of the water to somewhat less the that of natural sea water. "Hypo" comes from Greek and means under or below. For the treatment to be effective the salinity needs to be lowered to 12-14‰ which is equivalent to a specific gravity of 1.009 (d20/20) at normal tank temperatures (26-28°C - 79-82°F). Normal sea water has a salinity of 35‰ (Gross, 1977).

How does it work?

For the treatment of "Ich", hyposalinity works by breaking the life cycle of the parasite. The lower salinity may also reduce stress, although there is no hard evidence to date to support this.

C. irritans is a ciliate protozoan found in sea water and it has a number of stages in its life cycle. Infective stages burrow into the skin and gills of the fish and form a protective outer covering of skin. Here they feed on tissue fluids and skin and grow. When mature, the parasite breaks out of the cyst and after some time as a free-swimming form encysts on any suitable substrate such as the sand or rocks and divides many times to produce the infective forms (Colorni, 1987). The infective forms must find a suitable host or they will die.

The total time from the mature parasite detatching from the fish and reinfection of the fish is about 2 weeks at normal tank temperatures. This is why "Ich" may appear to clear up but then comes back a a week or so later but a lot worse.

The low salinity causes the most of the tomonts to rupture, killing them

Marine teleost fish (higher bony fishes) maintain their osmotic concentration at about one quarter to one third that of sea water. In normal sea water, these fish have a tendency to lose water from their gills due to osmosis and also in their urine. Fish have to drink a lot of water to make up for the loss, however, as the water contains a lot of salt (35‰) they must remove the excess salt from their system. The sodium and chloride ions are secreted by the gills and magnesium and sulphates are excreted in urine. This is an active process and requires energy much like the energy required to keep warm blooded animals warm.

When fish are under stress, one of the processes that is affected is ion regulation. This means they have difficulty adjusting the concentration of ions (sodium, chloride, etc.). Lowering the salinity of the tank water makes the concentration of ions closer to that of the fish"s internal fluids and reduces the fish"s efforts to maintain the correct concentrations.

Please note that only the higher bony fishes have lower osmotic concentrations and can be treated this way. Marine invertebrates have the same osmotic concentration as the surrounding water (Schmidt-Nielsen, 1975) and if left in the aquarium during hyposalinity treatment are most likely to die due to osmotic shock. Sharks and rays may not survive hyposalinity due to their unique method of osmoregulation. They have similar concentrations of salts to that of marine teleosts (one quarter to one third that of sea water), however, they also have very high concentrations of organic compounds which gives their internal fluids the same osmotic concentration as sea water. While some can adjust to lower salinities, most will succumb to osmotic shock just like invertebrates.

Where do you treat?

The first choice you must make is whether to treat in the main display tank or in a quarantine tank. The main display tank can be treated if it is "Fish Only" and contains no invertebrates or live rock. The live rock itself will survive the treatment, but any invertebrates living on and in it will be killed. If the tank is truly "Fish Only" treating the tanks is probably the best option as there will be less stress to the inhabitants and it helps you to eliminate the "Ich" in the main tank.

If the main display tank is "Fish Only With Live Rock", it may be possible to move the live rock to another tank and treat the main tank, but in most cases and when the tank is "Reef" tank, removing the fish to a quarantine tank is the only option.

When a quarantine tank is to be used, it is wise to fill the quarantine tank with water from the main tank when the fish to be treated are moved. This will help reduce the stress of the move and alleviate the acclimation process. It may still be necessary to acclimate the fish if time has passed from when the water was transferred and the fish are transferred.

How do you treat?

It is very important that you can accurately measure the salinity or specific gravity of the water. Cheap hydrometers, especially the swinging arm variety, do not have enough accuracy. If the salinity is too low, it is possible the health of the fish will be compromised. If the salinity is too high, it may have no affect on the parasites, as discussed above. A refractometer is the safest method for measuring salinity. If you don't have access to a refractometer, a lab grade floating glass hydrometer should suffice.

The goal is to granularly reduce the salinity of the water to between 12 and 14‰ and leave it at that salinity for at least 4 weeks but preferably 6 weeks. Basically continue the treatment for at least 4 weeks after the last spots disappeared.

The salinity must be lowered gradually to give the fish time to adjust to the lower salinity and more importantly ensures the bacteria in the biological filter can adjust. It should take around 2 days to get from 35‰ to 14‰.

Your starting point should be between 1.025 and 1.027. Replace about one fifth of the volume with RO, RO/DI or aged freshwater that has been well aerated. Repeat this 12, 24 and 36 hours later, monitoring the specific gravity along the way. After the fourth water change the specific gravity should be 1.010 or pretty close. Wait a few hours to make the final adjustment to get down to 1.009. Note that you can estimate the resulting specific gravity. If you are changing one fifth of the water and the current specific gravity is 1.025 the result will be:

Then, after 12 hours: After 24 hours: After the 4th change: Water temperature influences specific gravity and if you heat water without changing the salinity the specific gravity will decrease. As the goal is to keep the salinity between 12 and 14‰ it is important to know the temperature as well as the specific gravity.

What temperature should you use? Some people have argued that raising the temperature is good because it speeds up the life cycle of the parasite. While this is true, the elevated temperature also raises the metabolic rate of the fish causing increased oxygen and energy consumption and somewhat negating the benefits of the hyposalinity. Temperatures close to "normal", those the fish are used to, will be the best. Ensure the temperature is taken into consideration for the specific gravity.

While the fish are being treated it is extremely important to closely monitor the pH and specific gravity. Unless you are treating in a well established tank with an established biological filter there will be a tendency for the pH to drop and this must be monitored. This can also happen in an establish tank due to the lower salinity. If the pH starts to drop, water should be changed or buffer very carefully added to the system. If the fish are sick, too rapid changes in pH will not be beneficial.

The specific gravity also must be monitored as excessive evaporation will cause the salinity to rise and possibly create suitable conditions for the free swimming parasite. If the salinity does rise, it may be necessary to extend the length of the treatment after the salinity has been lowered again.

Hyposalinity should be maintained for at least 4 weeks but 6 weeks is preferable. If there is any reinfection of the "Ich" during the treatment, the treatment should be extended to at least 4 weeks after the disappearance of the last cyst.

When the treatment is complete, the salinity should be raised gradually to normal over a number of days. Water changes with normal or even high salinity water is the easiest way to get the salinity up. Calculations using averages, as before, can be used to determine the appropriate specific gravity of the replacement water. If you replace one sixth of the 1.009 water with water at 1.025, the specific gravity will be raised to 1.012:

As the salinity approaches normal it will be necessary to either replace more water in each change or use water with a higher than normal salinity. If you have been treating in a quarantine tank, you will need to leave the display tank with no fish in it for at least 30 days. This will ensures that all remaining parasites in the tank have gone through their full life cycle and the infective forms have died.

If you have been treating in the display tank, you should wait some time before returning any invertebrates or live rock to ensure that the biological filter has had time to adjust to the salinity and load.


Invertebrates, sharks and rays cannot tolerate low salinity and should be removed from the tank.

Summary Disclaimer

This information is presented in good faith to assist aquarists in the treatment of "Ich" and other conditions in their fish. The author accepts no responsibility for loss due to the use or misuse of the information.


Colorni A. 1987. Biology of Cryptocaryon irritans and strategies for its control. Aquaculture 67(1-2):236-237.

Gross, M. G. 1972. Oceanography: A View of the Earth. Prentice-Hall, Inc., Englewood, NJ, USA. 497pp.

Schmidt-Nielsen, K. 1975. Animal Physiology: Adaptation and Environment. Cambridge University Press, London, UK. 699pp.

Last updated: May 8, 2004