Aquarium Fish: Applications for Hyposalinity Therapy: The Benefits of Salinity Manipulation for Marine Fish

In recent years, hyposalinity therapy has become widely popular for the treatment of Cryptocaryon irritans or what is commonly referred to as “saltwater ich.” Hyposalinity therapy consists of manipulating the salinity of the water and maintaining marine teleost fish in a hyposaline environment. The salinity range for this treatment is generally 12-16ppt. This method gives aquarists an option to using copper-based medications or harsh chemicals to treat this common parasitic infection.

What is less well known are the other potentially beneficial applications for hyposalinity therapy with marine teleost fish. The most obvious of these is for the treatment of other types of external parasites, but there are more ways that it can be used. Hyposalinity can be employed in better acclimating recently transported fish, for quarantine, treating wounds, with antibiotics, getting fish to begin eating, conserving metabolic energy, improving growth and alleviating the effects of stress.

I am not suggesting that all marine fish be kept in hyposaline conditions indefinitely. What I am suggesting is being open to investigating the various potential applications for hyposalinity therapy.

There may be some concern that hyposaline conditions could be stressful to marine teleost fish, or otherwise potentially harmful. While this is true in extreme salinities, studies indicate that this is not the case in more moderate salinities that would be employed in hyposalinity therapy (Wu & Woo, 1983. Woo & Chung, 1995. McDonald & Grosell, 2006).


Natural Sea Water is much more saline than the internal fluids of marine fish. Because of this, they expend a considerable amount of energy to reduce the excessive salt load through the process of osmoregulation. The kidneys are not the primary site of electrolyte management in marine teleost fish (Stoskopf, 1993). Chloride cells in the gills excrete excess chloride and sodium. “The kidneys of marine fish do play a role in electrolyte excretion; however, there function is more important in the balance of magnesium and sulfate levels and not, as might be assumed, in sodium and chloride elimination” (Stoskopf, 1993).

There are a few precautions to take when employing hyposalinity therapy. Do not confuse salinity with Specific Gravity. An accurate refractometer or other device should be used each day to check the salinity. Plastic swing-arm type hydrometers are often too inaccurate for this purpose. The alkalinity should be kept up to prevent the pH from falling as it tends to drop in diluted saltwater. Check the pH on a daily basis. The salinity can be reduced rapidly when beginning treatment, provided the pH and temperature of the water do not differ from what the fish are used to. Take more time when raising the salinity back to normal after completion of treatment. You can raise the salinity a couple of points a day.


Acclimation and alleviating the effects of stress

The effects of stress caused by capture, transport and handling is a major concern when acclimating fish, especially when they have been bagged for a prolonged period. Stress affects fish in two ways: it produces effects that disrupt or threaten homoestatic equilibrium and it induces adaptive behavioral and physiological responses (Wendelaar Bonga,1997). Osmoregulatory dysfunction is closely associated with stress in fish. This is recognized by an increase in osmolarity in saltwater species (Carmicheal et. al, 1984. Robertson et. al, 1988.). This can manifest in the loss of up to ten percent of body weight due to dehydration in one or two days (Sleet & Weber, 1982.). Reducing the salinity gradient between the water and the internal fluids of fish is effective in counteracting osmoregulatory dysfunction and other physiological responses to stress (Johnson & Metcalf, 1982. McDonald & Milligan, 1997.) With marine teleost species, this is accomplished by reducing the salinity of their environment.

Quickly acclimating recently transported, or otherwise stressed marine teleost fish to low salinity water will help them to recover normal homeostasis more rapidly. Marine fish are most sensitive to changes in temperature and pH during the acclimation period. Match these parameters in the quarantine tank closely to the shipment water, provided they are not at levels that are dangerous to the fish. Then the pH and temperature can be adjusted slowly over a couple of days to match the display aquarium.



Placing fish in hyposaline conditions during the quarantine period is a proactive approach to dealing with some types of external parasites. Rather than waiting for fish to exhibit signs of infection, why not be one step ahead? This can save time, improve an animals odds of survival and help protect the established livestock in the display aquarium. Hyposalinity therapy is an excellent treatment for Cryptocaryon irritans
(saltwater ich). It can also be effective against some other types of parasites such as the Monogenetic trematode Neobenedenia melleni.

Hyposalinity is not an effective treatment for every possible external parasite that may be encountered. Be watchful for other possible parasites or infections during the quarantine period. I recommend maintaining the quarantine system, for a minimum of thirty days, at a salinity of 14ppt. A quarantine system with a matured biological filter will provide a much more stable environment than an un-cycled aquarium.




When marine fish have gill damage, open wounds, missing scales, or the mucus layer has been temporarily damaged, it places a heavy burden on the energy required to maintain osmotic balance. Wounds compromise the mucus/skin/scale barrier causing the efflux of fluids from the tissues of fish. This makes maintaining osmotic balance more difficult and costly energy-wise. If the fish are unable to keep up with the loss of fluids through osmosis it will lead to dehydration.

The loss of fluids can be counteracted by reducing the gradient between the internal fluids of the fish and the surrounding ambient water. For marine fish, this means simply reducing the salinity of the water. Employing a low salinity environment slows the loss of fluids due to
osmosis through the damaged water barrier, conserving energy that would otherwise be expended for osmoregulation. A salinity that is close to isosmotic would be the most effective: the internal salinity of marine teleost fish is generally 11-12ppt.

Compromises to the mucus/scale/skin barrier also make fish more susceptible to opportunistic pathogens, especially bacteria. Products that
contain polymers (polyvinylpyrrolidone or PVP) can provide a temporary layer of protection until the fish heal and the mucus layer recovers (Carmichael & Tomasso, 1988).



Some antibiotics work more efficiently in softer water than in full strength saltwater. For example: tetracyclines chelate divalent cations (calcium and magnesium). This inactivates tetracyclines and means that higher doses are required in hardwater (i.e. saltwater). All quinolones are also inhibited by high water hardness. Softer water (as compared to full strength saltwater) may help certain antibiotics to be more effective or allow them to be used in smaller doses.



Brooklynella hostilis kills badly infected fish quickly as it damages skin and gill tissues. The damage to the skin causes compromises to the mucus/scale/skin barrier making the fish more permeable to water. This leads to an efflux of water from the fish causing dehydration. Chloride cells in the gills are responsible for excreting excess chloride and sodium. Damage to the gills caused by brooklynellosis makes it increasingly difficult for fish to rid themselves of excess salts. The cause of death in brooklynellosis is the loss of epithelial which leads to an inability to maintain osmotic balance.


While hyposalinity is not a cure for brooklynellosis in and of itself, it can be a useful tool. Placing the fish in a hyposalinity therapy may buy some precious time. In a hyposaline environment, fish with damaged skin and gills will be better able to maintain osmotic balance. The energy cost for maintaining osmotic balance will also be reduced.

The most effective treatment for Brooklynella hostilis is a series of three formalin dips. The dips should be administered in three day intervals at a dose of 1ml/gal for 45 minutes. The dip should be well aerated at a temperature of less than 80F.


Getting newly acquired fish to begin eating sooner

Environmental conditions such as temperature, oxygen level and salinity affect the metabolism rates of fish. Their metabolism depends on the process of osmoregulation to provide a stable working environment.

Metabolism depends on

  • Nutrition and respiration for metabolites.
  • Osmoregulation to provide a stable working environment.
  • Excretion to remove useless or poisonous waste products

Digestion consumes metabolic energy. Since osmotic dysfunction goes hand in hand with stress in fish, we can expect that they will be expending a great deal of energy after a stressful event, such as handling, in recovering osmotic balance. This means that less energy is available for other functions such as digestion. There is a correlation between the resumption of feeding behaviors and the re-establishment of normal physiological status (homeostasis). Hyposalinity therapy will reduce the amount of energy required for osmoregulation, while decreasing the gradient between the internal fluids of the fish and the surrounding ambient water will speed recovery of osmotic balance.

Factors influencing feeding behaviors

  • Overall health
  • Water temperature
  • Security
  • Photo-period
  • Osmoregulatory balance



Conserving metabolic energy

Stress disturbs the normal physiological equilibrium or homeostasis of fish by forcing a reallocation of energy within its system. Fish survive
stress with the expenditure of energy. Any response or adaptation to stress consumes energy that could otherwise be used for maintaining normal body functions such as growth, digestion, osmoregulation, disease resistance, healing and reproduction (Barton and Iwama, 1991).

Energy is like a cake with only so many pieces and stress consumes a portion of that cake.

The functions of normal physiological equilibrium such as tissue repair, locomotion, respiration and hydromineral regulation (osmoregulation) take priority over the investment activities of reproduction and growth. Since stress and functions of normal physiological equilibrium take precedence over growth and reproduction, then conserving metabolic energy should improve these performance activities. Hyposaline conditions conserve metabolic energy that would otherwise be expended for osmoregulation in marine teleost fish.



Maintaining osmotic balance normally consumes 25 to 50% of the metabolic energy in fish. The hypothesis behind suggesting the use of hyposalinity is that conserving metabolic energy, in this way, may make a larger portion available for healing and recovering from illness. While conserving energy through hyposalinity therapy is not a direct treatment for lymphocystis, I believe that it is a potential aid.


Improving growth

Young fish require a lot of energy for growth. For marine species, the more saline the environment is the more energy is used in osmoregulation. Studies indicate that many species of marine fish exhibit improved growth at salinities that are close to isosmotic (the salinity of the surrounding water is close to the internal salinities of the fish). These studies suggest that raising these species in hyposaline conditions can be advantageous for aquaculture (Lambert, et. al., 1994. Gaumet, et. al., 1995. Deacon, N. &Hecht, T., 1999.) The increase in growth rates are the results of improved food conversion efficiency. All plasma concentrations (except chloride) were unchanged, suggesting that fish were well adapted to their environment. Oxygen consumption was significantly decreased in the 19ppt and 10ppt salinity groups (Gaumet, et. al., 1995.)

Besides improved growth, there is evidence to support the idea that hyposaline conditions may be beneficial to hatcheries. “Like several other marine teleosts, growth and survival of juvenile H. kuda tended to peak in diluted seawater salinities of 15 and 20 ppt” (Hilomen-Garcia, 2003.)


Treating Cryptocaryon irritans

Hyposalinity has several advantages over the use of copper or harsh chemical for treating Cryptocaryonosis in fish. Hyposalinity is a safe and effective alternative that is non-toxic and does not cause stress to the fish when used correctly. Copper suppresses immune function and it is toxic to fish. It is also an unstable substance in the aquarium so the level should be tested twice a day. Some antibiotics are not safe to use in
conjunction with copper. Carbon and chemical filtration pads cannot be used to maintain the water quality when using copper. There is also the problem of copper being difficult to remove from the aquarium after treatment is finished.

A salinity of 14ppt is recommended for treating Cryptocaryon irritans. This is an effective treatment the vast majority of the time. However it is possible to encounter an unusual strain that is resistant to low salinities. Treatment should continue for a minimum of three weeks, with thirty days being preferable. It usually takes a week or so for the telltale white spots to disappear. If the white spots re-appear then double check the salinity and make sure your refractometer is calibrated correctly.



Over the years I have authored articles on various subjects related to fish health management that are of special interest to me. I enjoy studying and writing about my hobby, especially subjects that I think have not been covered extensively enough. Some examples of these subjects are Cryptocaryon irritans, Beta glucan, updating acclimation procedures, stress in fish, metabolism, energy use and feeding behaviors. Although these subjects may not seem to be directly related, I began to see a relationship evolve. This has led me to new ways of thinking about fish health management and how all of these subjects actually intertwine to form patterns. Researching and writing about each subject, gave me a deeper understanding and appreciation for all of the others.

The various ideas that I shared in this article may or may not be new to you. If you are skeptical or wonder about any of them, then I welcome that. Being skeptical is a way of showing concern and I trust that everyone in our wonderful hobby is concerned about the subject of fish health management. I hope this will encourage you to read more books and articles including the references that go with them. Perhaps you will see some of the same relationships and patterns evolve. Perhaps you will come up with your own opinions, new thoughts, or ideas.


For further reading

  1. Metabolism, Energy Use and Feeding Behaviors in Fish
  2. Updating Marine Teleost Fish Acclimation Procedures: Part 1
  3. Updating Marine Teleost Fish Acclimation Procedures: Part 2



Osmoregulation: (process that controls the salt/water balance within fish) Pronounced: os·mo·reg·u·la·tion, The regulation of osmotic pressure. The control of the concentration of dissolved substances in the cells and body fluids of an animal.

Isosmotic: Pronounced: i·sos·mot·ic, with equal osmotic pressure. Chemistry relating to or exerting equal osmotic pressure.



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  2. Barton, B.A. & Iwama, G.K. “Physiological Changes in Fish From Stress in Aquaculture with Emphasis on the Response and Effects of Corticosteriods.” Annual Review of Fish Diseases, 1, 3-26, 1991.
  3. Carmichael, G.J. & Tomasso, J.R. “Survey of Fish Transportation Equipment and Techniques.” Progressive Fish Culturist, 50, 155-159, 1988.
  4. Carmicheal, G.J. Tomasso, J.R. Simco, B.A. & Davis, K.B. “Characterization and Alleviation of Stress Associated with Hauling Largemouth Bass.” Transactions of the American Fisheries Society, 113, 778-785, 1984.
  5. Deacon, N. &Hecht, T. “The effect of reduced salinity on growth, food conversion and protein efficiency ratio in juvenile spotted grunter, Pomadasys commersonnii.” (Lacépède) (Teleostei: Haemulidae) Blackwell Publishing, Aquaculture Research, Volume 30,Number 1, pp. 13-20(8), January 1999.
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  7. Hilomen-Garcia, G.V. Delos Reyes, R. Garcia, C. M. H. “Tolerance of seahorse Hippocampus kuda (Bleeker) juveniles to various salinities.” Journal of Applied Ichthyology 19 (2), 94–98, 2003. doi:10.1046/j.1439-0426.2003.00357.x
  8. Johnson, D.L. & Metcalf, M.T. “Causes and Controls of Freshwater Drum Mortalities During Transportation.” Transactions of the American Fisheries Society, 111, 58-62, 1982.
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    cod (Gadus morhua).”
    Canadian Journal of Fisheries and Aquatic Sciences [CAN. J. FISH. AQUAT. SCI.]. Vol. 51, no. 7, pp. 1569-1576.
  10. McDonald, G. & Milligan, L. “Ionic, Osmotic and Acid-Base Regulation in Stress.” In Fish Stress and Health in Aquaculture (ed. By Iwama, G.W. Pickering, A.D. Sumpter, J.P. and Schreck, C.B.), pp. 119-144. University Press, Cambridge, UK. 1997.
  11. McDonald, M.D. & Grosell, M. “Maintaining Osmotic Balance with an Aglomerular Kidney.” Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway, Miami, Florida, 33149-1098, USA, Feb 2006.
  12. Robertson, L. Thomas, P. & Arnold, C.R. “Plasma Cortisol and Secondary Stress Responses of Cultured Red Drum (Sciaenops occellatus) to Several Transportation Procedures.” Aquaculture, 68, 115-130, 1988.
  13. Sleet, R.B. & Weber, L.J. “The Rate and Manner of Seawater Ingestion by a Marine Teleost and Corresponding Water Modification by the Gut.” Comp. Biochem. Physiol. 72A, 469-475, 1982.
  14. Stoskopf, M.K. “Fish Medicine.” W.B. Saunders Company. Philadelphia, Pennsylvania, 1993.
  15. Wendelaar Bonga, S.E. “The Stress Response in Fish.” Physiological Reviews 77(3):591-625 July 1997.
  16. Woo, N.Y.S. & Chung, K.C. “Tolerance of Pomacanthus imperator to Hypoosmotic Salinities: Changes in Body Composition and Hepatic Enzyme Activities.” Journal of Fish Biology, 47, 70-81, 1995.
  17. Wu, R.S.S. & Woo, N.Y.S. “Tolerance of Hypo-Osmotic Salinities in Thirteen Species of Adult Marine Fish: Implications for Estuarine Fish Culture.” Aquaculture, 32, 175-181, 1983.
  Advanced Aquarist, Advanced Aquarist

 Terry D. Bartelme

  (16 articles)

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