After 26 years in the Koi hobby. After 25 years in the chemistry field. After reading every scientific study I can find on the subject. The only answer I can come up with to this question is ‘ignorance’.
People who add salt to a fresh water habitat do so with the genuine belief that it is benefiting their beloved pets. So they have to be ignorant of the facts, right?

Don’t shoot the messenger!!

26 years ago, when I was first sucked in to this amazing hobby, talk of adding salt to a pond containing koi was rife. The practice was widespread. Generations of koi keepers passed this ‘tip’ down and it was almost considered to be the norm. Today, the practice is much less prevalent. Social media, and the communication streams available to us today, mean that old wives tales such as this one can now be countered with actual scientific facts.

The last 26 years have been a journey of progression for the, still young, koi hobby. I have seen the koi hobby evolve and improve in every aspect. Filtration, pond design, treatments, food and, most importantly, knowledge and understanding. As such, practices such as adding salt to koi ponds because ‘that’s what my Dad did and it worked fine for him’ have been challenged rather than taken as gospel. Fact has replaced fiction, knowledge has replaced hearsay.

I see countries that are at an earlier stage on this hobby’s evolutionary journey to the UK where I live. I see them taking a familiar journey down a familiar path. Things becoming popular now that became popular here years ago and have since disappeared in favour of better options as we learned, often the hard way. Bottom drains, bog filters, fibreglass, black box filters, the list goes on and on.

Salt is another such practice that has become less popular in the UK as our knowledge has grown, and as purpose made treatments have been developed, but is still at its peak in other countries.

So here is the thing, ‘There is no proven benefit to a HEALTHY Koi from exposing it to salt’. There have been countless scientific studies carried out on this and, having read every one I can find, I can indeed confirm that not one benefit has been proven, for a healthy koi, that can be credited to exposure to salt.

Salt is a chemical, its chemical formula is NaCl or Sodium Chloride. Both Sodium and Chloride are particularly unpleasant elements.

The dangers of salt in a human diet is well documented. Its a chemical that, when used in food, enhances the flavour of pretty much everything. It supercharges the flavour of whatever we add it to. But it also needs to be used in moderation as too much leads to high blood pressure, heart attack and strokes.

Salt is a chemical that is alien to koi. By this I mean that they have evolved to function optimally in an environment that has no salt. Adding ANY chemical to an environment that is alien to its habitants is detrimental to their wellbeing. It is detrimental because the inhabitants of that environment are not designed and built to deal with said chemical. They therefore must adapt. Whilst they are adapting they are taking resources away from other systems. Appetite, immune system, growth etc are a few examples of the areas that could suffer due to the diverting of resources to adapt to the alien in their environment.
So whilst your koi may not directly be negatively impacted by salt, compromises are made which can and do have a negative impact. It could be the straw that breaks the camels back. It could lead to the small drop in immune system function that gives those few flukes the chance they have been waiting for to gain the upper hand.

Salt use in koi keeping takes two forms.

1. Short term to kill parasites

Salt is a chemical that can kill parasites. As I have already discussed, salt is a pretty nasty chemical. It is capable of killing living things. I wont be going in to to much science but suffice to say salt sucks the water out of living things resulting in death from dehydration. In high enough concentrations it can kill the parasites that take up home on our koi. Unfortunately, in these high concentrations it will also kill our koi given enough time. The good news is that, while unpleasant for koi, they will live longer than a lot of parasites in these high concentrations so as long as we are careful we can place a parasite ridden koi in a bath containing lethal concentrations of salt, leave it long enough for certain parasites to be killed, and then get the fish out and in to recovery before it to succumbs. This is a viable, albeit dangerous, option for parasite eradication. What I will say though is that, in the case of every parasite that koi can experience, there is a better alternative to salt. A safer alternative, an alternative that has less negative impact on koi. Less side effects, less of an ordeal for our fish. These treatments are developed by scientists and experts to be the most effective with the least side effects. Yes they cost more, but should be the first choice if at all possible.

2. Short term to relieve stress

The second use for salt in relation to koi keeping is as a short to medium term (a few weeks max) treatment to ease stress in a sick koi to enable it to focus resources where they are needed most. This occurs by easing osmotic stress.

Koi bodies, like human bodies, contain a level of salt. They are actually exactly the same in that they are both 0.9%. Water moves in and/or out of a koi’s body via something called osmosis. Osmosis is a term which in simple terms refers to the movement of a solution from one area in to another area because it has a higher concentration. Movement from lower to higher concentration. In a closed system this will happen until the solution in both areas are the same concentration and then once this balance is reached the movement stops. The two areas in question are separated by a semi permeable membrane through which the solution permeates.

The two areas we are concerned with here are the pond water and the koi’s body.

So given that we know a koi’s internal salt level is 0.9%, we know that if the pond water salinity is greater than 0.9% then water will move from within the koi, via the permeable membranes (body and gills) to the pond water. Under normal conditions, pond water is around 0.05% (tap water tends to have a small trace amount) and as such water is constantly moving from the pond water in to your koi’s body. Your koi’s organs regulate this to ‘balance the books’.
Whilst this process is automatic and perfectly normal, it does consume resources. As such, by temporarily removing the need to perform this process, or at least reducing the work load, we can take some work away from the koi and in turn free up those resources to use elsewhere, such as overcoming an ailment.

The key here once again is that there needs to be an ailment. ‘There is no benefit to a healthy koi form exposing it to salt’.

So if we raise the salt concentration in the pond when we have sick koi, say to 0.3%, we can take some of that work load off and help our koi in their battle with whatever is ailing them. Once again though, this is short term for sick koi and should be removed once the ailment in question has been treated and eradicated.

Unfortunately, as I alluded at the start of this article, there is a lingering train of thought in some circles that there is actually a third use of salt in a pond containing koi. That adding a level of salt to a pond all year round is somehow beneficial. I’ll start this section with this statement, ‘you can’t kill what’s not there’, take a moment to think about that.

If there are parasites present in your pond, you need to diagnose exactly which one(s) you are dealing with, and then treat accordingly to kill them. If you don’t have parasites in your pond, why on earth would you submit your koi to a harsh chemical treatment??? You can’t kill what’s not there!!

To this end, there is no such thing as ‘preventative treatment’. The prevention you need is spotless clean water, adequate filters which are cleaned regularly (preferably every day), good quality food, regular observation, oxygen and vigilance on your part.

In general, people who add chemicals to their ponds in the name of preventative treatment are doing so in an attempt to cover up for a lack of one or more of the above. If you do your job properly, you tick all the boxes above, you will do fine without chemicals. Your koi will flourish.

Those who add salt all year round are generally of the belief that the salt is killing parasites. It is not. Koi cannot live long in water that is high enough salinity to kill parasites. The salt levels in question are killing nothing at all (except some plants if you have them). It is doing nothing positive at all. It is, however, doing harm. As an alien chemical it is negatively impacting your koi’s appetite. So growth is reduced, but also, due to lower intake of nutrients from eating less, immune system and general health and wellbeing are also reduced.

Consider if you will the recent covid pandemic. We were all encouraged to get vaccinations. By having salt in your pond, at levels that are not high enough to kill parasites, you are effectively vaccinating them against salt. Giving them exposure to low levels so they learn to tolerate it and cope with its intrusion. They become better able to resist it should you actually need to use it to kill them.

Some people claim that salt helps koi to resist parasites by increasing mucus layer thickness. Most parasites live in the mucus layer of your koi so more mucus is not a deterrent at all, its not a physical barrier to a parasite. And while there is an element of truth in this, what is actually happening is that the salt is irritating your koi. It is uncomfortable for them to be in salt water. So their body tries to shield them from the irritation by increasing mucus production. This is not a positive as far as I’m concerned.

Salt is also proven to dull the colours of a koi’s skin. Long term salt exposure, even at very low levels, damages the pigment cells. Whether this damage is permanent or is repaired once the koi is removed from the salt solution is not known but suffice to say it’s best avoided.

Salt is incompatible with a lot of koi parasite treatments so by adding salt to the pond you are limiting what treatments can be used. It is also difficult and time consuming to remove once it is in your pond as you have to dilute it out.

Lastly, not all parasites are killed by salt. At least not at concentrations that koi can survive. There are some parasites that will out live your koi in salt solution, flukes for example. There are also some, Trichodina and Costia to name two, that actually reproduce more readily in salt. So faced with the choice, Trichodina and Costia would CHOOSE to have some salt in the pond!!!

I hope you found this useful. If so do feel free to share this article as you see fit. I only ask that you credit Dazzle Koi when you do. Dont forget to check out the rest of my website and my youtube channel for lots more helpful articles and videos related to this wonderful hobby.

Daz