Human Nature – Is UK Tap Water Safe For Koi?

koi tap water

You might not consider your health to be at high risk from tap water, but when it comes to your koi it can be a whole different story.

UK tap water is perfectly safe for you to drink – fact. Top limits are set for the substances found in tap water and these limits follow World Health Organisation (WHO) recommendations which are based on a whole lifetime’s drinking, not just a day, a week or a month or two. So gentlemen, if you forgot to give your partner red roses on valentine’s Day, don’t panic when you see arsenic listed amongst other things in your tap water – you are perfectly safe!

But, what is perfectly safe for you is not always safe for you koi carp. Humans are right at the top of the animal chain and those at the top of the chain have the highest tolerance levels. Fish are right down near the bottom of the chain and, in the main, those at the bottom can tolerate the least. So, it’s important to avoid taking the attitude that if your tap water is good enough for you then it is fine for your fish – it doesn’t work like that.

To bring that into context, we would consider that water used for human renal dialysis is pretty pure but, in fact, this water contains levels of chlorine, chloramine, and copper that exceed those that fish can safely tolerate.

A lot is said about fish needing essential minerals, so do we. However, what is at essential mineral or acceptable levels for us is, in some cases, far, far too high for fish safety. Very good examples of that are copper and zinc. See Figure one.

Figure one

So, for koi safety, it is often important to keep two opposite views in mind, about the same thing, at the same time

The differences demonstrated in figure one between top fish and top human safe values are, relatively speaking, huge. From these examples, you can see that a nominated amount of something could be described as being either at acceptable or essential mineral level for us and, at the same time, be at a potentially extremely damagingly high level for fish.

So, for koi safety, it is often important to keep two opposite views in mind, about the same thing, at the same time. See Figure Two

Figure two

Life has a habit of being contrary and the same can be said about tap water for fish. There are some things where the top limit allowed for people is actually below the fish safe value. Two examples are arsenic and atrazine.

According to G. post, Textbook of Fish Health, published 1987, the fish safe value for arsenic is probably less than or equal to 0.7mg/l (700 µg/L).

Figure Three shows this fish safe value in contrast to the top amounts allowed in UK and USA tap waters.

Atrazine belongs to the pesticides group. The top fish safe value is 10µg/L whereas in UK tap water it is only allowed to be up to a maximum of 0.1 µg/L.

Both arsenic and atrazine, in UK tap water, are therefore going to be way below fish safe values and can be way below fish safe values can be ignored, as they will not be present in sufficient quantities to harm the koi. Except atrazine is the most difficult of pesticides to reduce the purification terms, therefore it is a good overall test to see how well purifiers reduce other substances in the pesticide group.

So can substances in tap water harm fish?

Yes they can. Different groups of substances in tap water can harm koi in various ways. And it is important to divide them into groups to understand the effects they can have on your pets. In my view, koi are treated with unnecessary medication sometimes.

That is because the symptoms are often being treated rather than solving the root cause. If you do not solve the cause of koi health problems then you are at risk of them re-occurring. That can apply to dirty filters or partially blocked bottom drains, fence treatments etc, as well as substances in tap water.


Disinfectants are needed in tap water for bacteria control and to prevent the population suffering such things as dysentery and typhoid. The two disinfectants in tap water are free chlorine and chloramine. Unfortunately, they attack gills and are harmful to koi, see Figure Four.

Organic Compounds

Organic compounds are related to industrial waste and such things as detergent and paints. They are more likely to be at koi damaging values in the pond water because of a koi keeper’s own actions. For example a use of organophosphates as medications. Although this area of water pollutant is more likely not to be aware of effects they can have on koi, see Figure Five.

Clearly we ignore substances in water to our and our koi’s peril

Figure Five

Already you can see that substances in water can have a marked and in some instances, a drastic effect on koi health.

Clearly we ignore substances in water to our and our koi’s peril. Obviously not all pollutants are going to be at high enough values to cause koi death. Sometimes they will be at a level where gradually and slowly they chip away at koi good health, at other times the values will be acceptable and will cause no problems for the fish at all. In worse case scenarios substances can be at levels which damage, and possibly kill, koi in a relatively short time. It is like the difference between black and white and all the shades of grey in between. Only by investigating the water your koi are trying to survive in, can you possibly know what effect the ‘water’ may be having on them.


This is a generic group of substances consisting of algicides, fungicides, herbicides, insecticides, molluscicides, in America and other hot countries termiticides and of course, pesticides themselves. They are grouped together because they react similarly when planning water treatment programmes. Some are highly toxic to koi and need to be kept at very low values. For examples see Figure Six.

If you do not solve the cause of koi health problems then you are at risk of them re-occurring.

Water & Health

Figure six

Permethrin is a molluscicide. That is, it kills freshwater shrimps and snails. It is also highly toxic to fish and I would prefer to see none present at all in pond water. For the effects of pesticides on fish, see Figure Seven.

Figure seven

Damage to the central nervous system is important as it can lead to such health problems as strokes. Damage to the liver and kidney can effect essential metabolic processes. Damage to the kidney can also lower the production of red blood corpuscles thus risking a potential problem that a fish may develop both anaemia and a lowered immune system.

Toxic metals

Although iron particles (carbonates, bicarbonates and oxides) can coat fish gills and prevent a transfer of oxygen across the gill membranes, it is mainly the dissolved metals which are harmful to fish. Toxic metals are interactive (together they are more harmful) and the effect of aluminium is greater in waters which have a high or a low pH. It is also known that younger fish are more susceptible to toxic metals damage than older, mature fish and that a sudden influx can cause a pond wipe-out. For toxic metals damage to fish, see Figure Eight.

Figure eight

“If you do not solve the cause of koi health problems then you are at risk of them re-occurring”

Wounds happen more easily and wounds fail to heal because of the weakening effect of toxic metals on skin and scale. With a failure of a fragile and damaged skin to heal there is a risk of ulcers forming. The ulcers (open wounds) provide opportunistic bacteria an entry point leading to bacterial infections developing in a koi.

Tap water can also be too soft for koi, this is a huge subject in its own right.

Un-adapted tap water and koi don’t always mix well, often to the detriment of a koi, while successful intervention can be to their advantage.

Leave a Response