Salt, a good general disease treatment.
Salt has long been the fish-keeper's treatment standby. Salt works
well against many protozoan parasites such as Costia, Trichodina and
Chilodonella as well as flukes and other ectoparasites. It can also
assist osmoregulation problems caused by bacterial ulcers; help clear
congested gills as well as supporting fish suffering from stress.
Because it works in a different way to most disease treatment, it is
safer than many pond treatments and will not adversely affect biological
filtration in pond filters. It is generally used at fairly high rates in
short-term baths or dips, but can be used as a long-term supportive
treatment in ponds.
Its action is based on altering the osmotic gradient between the
parasite or fish and the surrounding water. Osmosis, as you will recall,
is the movement of water from an area of high concentration (where the
water contains less dissolved substances and therefore more free water
molecules) to an area of less concentration (where the water contains
more dissolved substances and fewer free water molecule).
This is a difficult concept to grasp and explain, but it means that
in a freshwater pond or tank there is a continuous movement of water
from the pond water into the parasite's body fluids which contain
dissolved molecules such as proteins, salts, ions etc. Most aquatic
organisms such as fish and parasites control this continuous influx via
osmoregulation – literally regulating osmosis. (see water quality
articles for more info). The details are not important as long as we
understand that osmosis takes place and that in freshwater, water is
being continuously drawn into fish and other organisms.
organism's body fluids have high levels of dissolved substances
- proteins, metabolites and salts. These bind water molecules
and effectively reduce the number of 'free' water molecules.
Whereas the surrounding water has relatively small amounts of
dissolved substances - mainly inorganic molecules such as
calcium etc. and therefore larger numbers of free water
molecules to diffuse across a semi-permeable membrane. This
difference in concentration causes water to be drawn, by simple
diffusion, into the organism's body.
osmoregulation diffusion would continue until a state of
equilibrium was reached and the organism's body fluids had the
same concentration of free water molecules as the surrounding
When salt is
added to the water this attracts water molecules and reduces the
number of free water molecules. If the total sum of solute
molecules in the surrounding water exceeds that of the body
fluids, then water will be drawn, by diffusion, out of the
important implications for the importance of water hardness.
Hard water will bind more water molecules than soft water and
therefore exert less osmotic pressure than soft water. Water
ingress into fish with ulcers will be less in hard water than
OK. That’s the difficult part – now for the clever bit. If we
reverse the osmotic gradient by adding salt to the water, the osmotic
flow will reverse. So now, instead of water being drawn into the
parasite, it will now be drawn out, dehydrating the organism.
Clever! Clearly, smaller organisms such as flukes, will be affected
quicker and more severely than a large fish such as a koi. In addition
to controlling parasites, short-term baths have a mild astringent effect
and and help clear congested gills by lifting away cellular debris and
Helps bacterial ulcers and stress.
One of the effects of stress is to interfere with osmoregulation. A
longer-term bath of a low concentration (5 grams per litre) can help
alleviate osmotic stress by reducing the osmotic gradient and thus the
water flow into the fish. This is particularly important in soft water
because it contains less dissolved substances (and therefore has more
free water molecules), and thus creates a stronger osmotic effect than
hard water. Along the same lines, the same effect occurs with bacterial
ulcers. Because ulcers are literally holes in the skin, water can flood
into the fish’s body, again particularly in soft water. Reducing the
osmotic gradient by adding a small amount of salt to the water will
reduce water ingress.
1-3 grams per litre[(1-3 ppt - 0.5%)(0.25 - 0.5 oz per gall
UK)]. The lower dosage is often recommended for koi ponds as a
prophylactic treatment over winter or on a permanent basis. There
is little evidence that such a regime will prevent parasite
problems. Such a treatment would be useful if treating bacterial
ulcers, when a slightly higher dose of 5 grams per litre should be
Bath treatment for
freshwater ectoparasites and bacterial gill disease.
10-30 grams per litre[(10 - 30 ppt - 1-3%)(1 - 4.7 oz per gall
UK)] for up to 30 minutes. The higher dose may only be tolerated
for a few minutes. My own preferred dosage, unless there is a
reason to go higher, would be 20 grams per litre (3oz per gall UK)
for 20 minutes.
Does and don’ts
Make sure it is fully dissolved to prevent ‘salt burns’
Aerate water in short-term baths
Remove fish if they ‘go over’ as weaker ones may not stand
the full treatment time
Use cooking salt. Do not use salt containing anti-caking agents
such as sodium ferrocyanide (yellow prussiate of soda – this can
release hydrogen cyanide when exposed to sunlight!)
In the absence of a full examination, a salt bath is a good first
choice treatment for fish that are off-colour. For a variety of reasons
it is not always effective. If the fish doesn’t respond, further
investigation may be required. If you don’t have quarantine
facilities, salt-bathing new fish is a wise (and minimum) precaution
before putting them in the tank or pond.
i.e. 5 ppm = 5 mg / litre
mg / litre x
3.785 = mg / gall
i.e 5 mg / litre = 18.9 mg / gall (US)
mg/ litre x 4.546 =
mg / gall
i.e 5 mg / litre = 22.7 mg / gall (UK)
To convert imperial
gallons to US gallons multiply by 1.2
1 ounce = 28.35 grams
1% solution =
10 ml per litre
10 gram per litre
38 gram per
45 gram per gall