About Hyposalinity or Osmotic Shock Therapy

A Simple, Effective, Non-Chemical Treatment for Ich

Tetra in fish tank
Photography by Peter A. Kemmer / Getty Images

What is hyposalinity?  Hypo means "lower than normal", and saline/salinity "of or containing salt", therefore hyposalinity in the simplest of terms is a lower amount of salt contained in seawater than normal. With the salinity level of oceans and seas around the world averaging out at around 1.024 (specific gravity), although lowering the amount of salt in water by a few points might be called hyposalinity, when it comes to saltwater aquariums this means bringing the salt content down to a range of 1.013 to 1.010 (specific gravity) to be effective and truly considered as such.

How Does Hyposalinity Effect Ich and Other Marine Organisms?

All marine creatures require freshwater just as we do to survive, they just process it differently. Since their bodies are less salty than the water surrounding them, to prevent the loss of needed freshwater they take in seawater, process it to eliminate the salt molecules, and then retain the freshwater to maintain a balance with nature. When the salinity of seawater is lowered, or hyposalinity is applied, it results in a lowering of the osmotic pressure of the water at the same time, thus the related name Osmotic Shock Therapy (OST). Fish and a few other sea creatures can withstand and adjust to this change in pressure, but protozoan (Cryptocaryon/White Spot Disease, and Brooklynella/Clownfish Disease), dinoflagellate (Oodinium/Velvet or Coral Fish Disease), and flatworm (Black Spot Disease) ich organisms cannot. Reduce this necessary pressure, particularly rapidly, and they literally explode! Although delicate corals and invertebrates may not immediately rupture as ich parasites do, these too are marine animals that cannot tolerate exposure to low osmotic pressure, resulting in a rather quick death.

When Is Hyposalinity Most Effective on Ich?

Hyposalinity is largely ineffective on mature ich parasites that are well protected in the gills surrounded by thick mucus produced by an infected fish when embedded deep in the tissues of their host and during the final encrusted cyst stage of life. 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.

When Can This Method of Treatment Be Used?

Applying hyposalinity or osmotic shock therapy to treat ich problems is a personal choice decision one has to make, but here are many of the ways it can be used.

For a QT

  • While treating fish in a QT, lowering the salinity can help prevent newborn ich organisms that might be released from mature cysts from reinfecting the fish during the quarantine period.
  • As a preventative measure, when new fish are brought home and placed in a QT for several weeks of observation before introducing them into the main aquarium.

For Fish-Only Tanks

(Note: Under the following situations it is recommended to at least remove and give all exposed fish a freshwater dip, 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.)

  • When a QT is not available, or the choice is made not to treat ich infected fish in one.
  • When one does not want to leave their aquarium empty with no fish to look at for a month.
  • When there is a concern or one wants to lessen the possibility of re-infestation occurring after the fish have been treated in a QT and returned to the main aquarium.
  • When re-infestation does occur after the fish have been treated and returned to the main aquarium.

For Reef Tanks

Hyposalinity should NEVER be used in a reef system, as it will kill corals and all types of delicate invertebrates. Since most people in all likelihood will not want to disturb these animals, not to mention have to hassle with removing them and set up another tank to put them in, the easiest thing to do for 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