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

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

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

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,


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

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

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


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

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

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.,

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

A salinity of 14ppt is recommended for treating Cryptocaryon
. 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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    .” Annual Review of Fish Diseases, 1, 3-26,
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  7. Hilomen-Garcia, G.V. Delos Reyes, R. Garcia, C. M. H. “Tolerance
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    119-144. University Press, Cambridge, UK. 1997.
  11. McDonald, M.D. & Grosell, M. “Maintaining Osmotic Balance
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    Several Transportation Procedures
    .” Aquaculture, 68, 115-130,
  13. Sleet, R.B. & Weber, L.J. “The Rate and Manner of Seawater
    Ingestion by a Marine Teleost and Corresponding Water Modification by the
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  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
    .” Journal of Fish Biology, 47, 70-81, 1995.
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  Advanced Aquarist

 Terry D. Bartelme

  (16 articles)

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