Hypo means "lower than normal" and saline/salinity "containing salt," therefore hyposalinity in simple terms means a lower than normal amount of salt is contained in the seawater. The salinity of seawater can be measured as Specific Gravity (SG) using a hydrometer or refractometer. Specific gravity is the measurement of density of liquids when compared with pure water, which has a specific gravity of 1.000. With the salinity level of oceans and seas around the world averaging at around 1.024 specific gravity, lowering the amount of salt in the water by a few points might be called hyposalinity. However, when it comes to saltwater aquariums, this term means bringing the salt content down to a range of 1.010 to 1.013 SG to be effective for parasite control. When reducing the salinity, be sure the pH and temperature of the water do not differ from the current water conditions for the fish.
The Effects of Hyposalinity
All marine creatures require water (as we also do) to survive, they just process it differently. Since their bodies are less salty than the water surrounding them, saltwater is constantly entering their bodies by osmosis through the gills or skin, as well as by ingestion through the mouth. To prevent the loss of needed freshwater within the body, they eliminate the salt molecules through special glands in the gills, and then retain the freshwater to maintain their body fluids with lower salinity than the seawater. When the salinity of seawater is lowered, or hyposalinity is applied, the decreased osmotic pressure of the water causes tissue cells to expand with water, thus the related name: osmotic shock therapy (OST).
Fish and a few other sea creatures can withstand and adjust to this change in osmotic pressure, but protozoan (White Spot Disease, and Clownfish Disease), dinoflagellate (Velvet or Coral Fish Disease), and flatworm (Black Spot Disease) organisms cannot. Reducing this osmotic pressure, particularly rapidly, will cause them to take in water and they literally explode! Be aware, however, that delicate corals and invertebrates may not immediately rupture as ich parasites do, but these marine animals cannot tolerate exposure to low osmotic pressure, which could result in a rather quick death.
When Hyposalinity Is Most Effective
Hyposalinity is largely ineffective on mature ich parasites that are well protected within the gills or skin, surrounded by epithelial cells and thick mucus produced by an infected fish. This occurs when they are embedded deep in the tissues of their host during the feeding stage and during the final encysted replication stage. It is primarily during the free-swimming phase of life when newborn organisms are released from a mature cyst, and before they have the chance to fully attach and develop into mature parasites, that they are most vulnerable and can be eliminated with hyposalinity.
Applying hyposalinity or osmotic shock therapy to treat parasite problems is a personal decision one has to make. However, it is often safer to use than the medications that are added to the water to treat parasites on fish.
Treating fish in a quarantine tank (QT) with lower salinity can help prevent newborn ich organisms released by mature cysts from reinfecting the fish during the quarantine period.
As a preventative measure, place new fish brought home in a QT for several weeks of observation before introducing them into the main aquarium. Use hyposalinity for part of the time during quarantine to reduce the parasites on the fish.
Under the following situations, it is recommended to at least remove and give all exposed fish a freshwater dip, which is a short hyposalinity treatment. This is preferably in combination with at least a one-time appropriate medical treatment before placing the fish back into the main aquarium (at which point the salinity is lowered for 3 to 4 weeks). Use a freshwater dip when:
- A QT is not available, or the choice is made not to treat ich infected fish in one.
- You don't want to leave their aquarium empty with no fish to look at for a month.
- There is a concern or you want to lessen the possibility of re-infestation occurring after the fish have been treated in a QT and returned to the main aquarium.
- Re-infestation does occur after the fish have been treated and returned to the main aquarium.
Hyposalinity should NEVER be used in a reef system, as it will kill corals and delicate invertebrates. Since most people in all likelihood will not want to disturb these animals, not to mention the hassle of removing them and setting up another tank to put them in, the easiest thing to do for treating a reef tank is to leave it devoid of all fish for at least 4 weeks and allow the ich to run its life cycle and die off, while treating all the fish in the Quarantine Tank.
Cryptocaryon irritans Infections (Marine White Spot Disease) in Fish. University of Florida Institute of Food and Agricultural Sciences, 2020