test agua

Colombo Test for water parameters of your koi pond

“How are the parameters of your water?”

That will always be the first reaction if someone contacts us when they have problems in their pond.

“Well …. the pH is at 7.8. “

This answer is acceptable for a person who starts in the world of koi.

But for those who have been around for a long time and those who APPARENTLY care about their koi, this answer is absolutely unacceptable.

In the article “Water Management for Koi Fishes” we stress out that BEFORE becoming a koi owner, you have to become a water manager.

Testing the water in your koi pond is fundamental to the health of your koi.

Here we present a complete set: Colombo Test.

Colombo Test

Koi Hacienda: Colombo Test

It is a complete set to test the water in your koi pond:

  • pH – acidity
  • KH – temporary hardness
  • GH – permanent hardness (or general)
  • NH3 – ammonia
  • NO2 – nitrites
  • NO3 – nitrates
  • PO4 – phosphate

Most of the multi-tests found in the market contain many of the parameters mentioned above, but not ALL. Colombo Test contains ALL the important tests to control the water in your pond.

Apart from being a complete set, it is an intelligent test: The most commonly used test fluids can be purchased separately.

Colombo Test, to test the water in your koi pond, is presented in a very professional way in a flat and small plastic suitcase. All of us who have worked with the individual tests, know that after a certain time, the cardboard packaging is breaking, especially when a drop falls onto it or by leaving it near the pond in a wet area.

Another disadvantage of traditional tests is that the cards to compare the results obtained may be lost. Especially if the packages no longer exist.

With the Colombo Test you have it all together in a practical suitcase. You do not have to worry about where the test or the color card is, because everything is always together.

estanque koi agua

Koi Hacienda: Colombo Test in a handy case

The most frequent parameters NH3, NO2 (ammonia and nitrites) and pH are very easy to measure: 5 ml (for ammonia 10 ml) of pond water + 5 drops of the corresponding liquid.

For the KH and GH (the two kind of hardness) the procedure is different: The drops must be added and counted until the water changes color.

And for nitrates (NO3) and phosphate (PO4), in addition to the drops, you have to add the corresponding powder.

Maybe it sounds a bit confusing, but it’s very easy, as the following video shows.

Testing the water in your koi pond is no longer a nightmare. With Colombo Test you always have everything at hand.

Now you no longer have an excuse to not have all the parameters at hand when we ask you 😉

testar agua estanque koi

Koi Hacienda: Colombo Test – handy test case

Gestión y Mantenimiento de Agua para carpas KOI

Water Management for Koi Fishes

Water is the living environment of our Koi. Stable water quality is a key feature of successful water management and has its effects on the Koi health and well being.

The water quality has a direct influence on the growth, the colour, the reproduction and the life time of the Koi.

It is our duty to keep their well-being on a high level. We know that the pond water must be at an optimum for the Koi to live in, but what is a good water quality? Clear water is not always an indication of purity; it may contain dissolved waste such as nitrite and ammonia. The water quality is stipulated by its temperature, hardness, oxygen level, pH etc. All these values are related to each other. Each has its specific role in the water and its implications on the fish’s health and the bacteria.

Pure water is colour- and odourless. It has a great importance for all creatures because of its many functions in the living organisms like thermoregulation, transport of nutrients, dissolution of minerals etc… Water is the extension of the body liquids.

Monitoring and controlling the water quality is a crucial aspect in Koi keeping. Sudden changes or a chronic bad water quality will change the chemical balances and can be toxic for the fish, and causes stress.

The water must be checked regularly because measuring gives us the knowledge about the water quality and the possibility to correct what is going wrong. If you have the right attitude, then it is no work at all, you can react very efficiently and quick. Most problems can be overcome by the simple addition of good, fresh water.

Keep a record of the measured values over the time.

You can measure all these parameters with sticks, drops, digital devices or through continuous monitoring.


We have to prevent big changes in the water, there has to be sufficient oxygen and a minimum of waste in the water. The parameters should be watched and measured on a regular basis.



Water temperature min. 2 °C -max. 35 °C
pH 6,8- 8,5
GH min. 8° dH – max. 25° dH
KH 4-12°dH
O2 min. 7 ppm through 15 ppm
NH3 max. 0.1 mg/l
Redox between 350-400 mV
NO2 max. 0,1 mg/l
NO3 max. 50 m/l
CO2 between 2 and 5 mg/l, max. 25 mg/l


The water temperature in a pond is influenced by the season of the year and the weather.

The water temperature has a direct influence on our fish. The body temperature of a Koi is only 0.5 °C different fro his environment, the water… The water temperature influences the development and growth of our Koi.

Koi carps are basically very enduring fish, being able to survive at a water temperature range from 2 °C through 35 °C. The most ideal temperature is between 22 and 28 °C, when the Koi metabolism works at its optimum. Above 28 °C, the metabolism will slow down as the food is less well digested.

The water temperature is also important for the filtration. The bacteria work at their best at temperatures between 18 and 28 °C. Below 10 °C and above 32 °C they are inactive.

In warmer climates the heat can cause problems for the oxygen level in the water, the Koi will suffer from it if you don’t aerate the water.

If you change the water temperature too quickly, especially from warm to cold water, and this is more than 5 °C a day, the Koi will suffer and get stressed. The little ones may even die! Big temperature differences happen especially in small ponds with little depth.

During winter the Koi go into hibernation below a temperature of 8 °C. They barely move anymore, are huddled together on the bottom of the pond and refuse food. They go into a “torpid” state. The metabolism is at its lowest function…

Water Management for Koi Carps

PH = acidity

The pH or acidity is an abstract value. It means “potentia hydrogenii” or the “power of hydrogen”, and was defined in 1909 by Soren Sorensen.

The pH is the concentration of free H+ ions in the water. A water molecule H2O consists of H(ydrogen) and O(xygen).

The pH scale ranges from 0 (very acidic) to 14 (very alkaline); clean water has a pH of 7, which is neutral. 1 point counts for 10, meaning +1 pH-degree = a ten times higher concentration. Two degrees of difference make water 100 times more acidic or alkaline. Never underestimate a fluctuation or a change in the pH value!

0 very acidic
1 hydrochloric acid
2 stomach acid
3 vinegar
4 tomato juice
5 acidic rain
6 urine
7 neutral/fresh water
8 sea water
9 baking soda
10 soap
11 household ammonia
12 bleach water
13 sodium hydroxide sol.
14 Very alkaline/tube cleaner (Destop)

The ideal pH for our Koi is between 6.8 and 8.5. It is no use to struggle to keep the ideal value, it’s the big changes you have to avoid because sudden changes will damage the skin and gills of the Koi, even cause acute death!

A big change in the pH value has big consequences for the ion balance in the water. It has connections with different factors like the concentration of salts, acids, bases, ammonia and nitrite.

After a rainstorm the pH can crash for different reasons such as acidic rain or lots of waste in the water; the buffer KH will go down and the pH will lower too quickly.

The biological working of the filter diminishes, the bacteria die, and they are polluting the water, which will result in more ammonia in the water because of the destroyed system. The fins of the Koi will be reddened; the fish are stressed and will be flicking or jumping out of the water to escape the environment.

Thus you need to change the water! But, by doing so, the pH will rise again, which will alter the ammonia again! You can fight this by adding Zeolith to the filter or by adding sodium bicarbonate (= baking soda).

Too high a pH comes from the calcium in the water. Here in Spain, the pH of tap water can be very high.

You can lower the pH by adding peat, well water or acids.

When there are a lot of algae or plants in the pond, the pH will vary a lot. Algae and plants produce CO2 during the night (together with Koi); the pH is low in the morning… During the daytime, this CO2 is used by the plants and algae, the pH rises. If the difference between morning and evening is more than 1 unit of the pH, it becomes stressful for the Koi. Nowadays, there are devices which add automatically hydrochloric acid to the water until the right pH is reached.

Good aeration of the pond at night will expel the CO2 from the water and will make the pH stable again.
Measure the GH, it has to be between 8 and 12° dH. At these values, the free molecules of CO2 can unite with Ca (calcium) and form carbonates, which will keep the pH stable.

You have to know the value of pH when you administer medicines!

Oxigen and Water Managemente for Koi Carps


The KH is important for buffering the pH in the pond.

It is the product of the free CO2 gas and salts of calcium and magnesium. It represents the quantity of dissolved CO3– and HCO3– and is expressed as °dH.

In terms of hardness our Koi can cope with a wide variation.

The ideal value is above the 4° dH, i.e., between 5 and 10. In Japan, the hardness is only between 1 and 4° dH. There the big volumes of water act like the needed buffer. They use oyster shells to increase the KH.
In fresh water, the salinity (= the level of the total of dissolved salts) is relatively low compared to sea water. The hardness depends on the concentration of calcium and magnesium salts.

If the KH is high enough, it acts like a stabilizer for the pH. As long as there is enough bicarbonate in the water, the H+ will be neutralized and the pH remains stable.

The KH influences the osmosis regulation system too. It reduces the pressure on this system. Some medicines are more toxic in soft water or less efficient in hard water…

You can maintain the KH level by water changes and the aeration during night time. If you don’t add oxygen, the level of KH will go down because of the acid that comes from the nitrification process.

A low KH will result in abundant algae growth while the water plants don’t grow. The pH will be very high.

To raise the KH you may add NaHCO3 (sodium bicarbonate = baking soda), 30 g/m³ will alter the KH by one unit! Put pond substrate on the bottom and make sure the GH is at least 8° dH.



This is the amount of calcium and magnesium salts in the pond. It is also expressed in ° dH, but it is totally different from KH. They can change independently from the other.

GH plays an important role in the vital functions of the cells of the Koi, the osmotic working, plants and micro organisms. In soft water, the Koi uses more energy to maintain the balance of salt/water. Very hard water will result in kidney stones and increases the risk to leftover eggs.

A low GH will give a faster growth and better development of the red colour, while the black colour will develop slower. In Japan, the GH is situated between 2-3° dH.

0-4° dH very soft water
4-8° dH soft water
8-12° dH medium hard water
12-25° dH hard water

Above 25) dH, you have very hard water!

A correct GH will let the nitrifying bacteria and the plants thrive. The free CO2 will be bound by calcium, which will result in an ideal KH. This, in turn, will stabilize the pH.
You can alter the GH by adding minerals to the water. Lowering is more difficult; you can only do it by adding rainwater or through osmosis (very expensive).


Air is one of the most important substances for life.

Air consists mainly of nitrogen (78 %) and oxygen (21 %). The rest consists of CO2 and noble gasses like argon. The oxygen level in the air is 30 times higher than in water.

O2 was discovered by the Scheele in 1771.

It is a colourless gas, without taste or smell, and it consists of two oxygen atoms which adhere very easily to other molecules. O2 is formed through photosynthesis of green plants (also algae) during the daytime. Plants use sunlight as an energy source to transform CO2 and water into glucose and oxygen.

Glucose (sugar) is a nutrient for plants, while the oxygen will be used by bacteria and fish to burn the glucose for energy production. In daylight, the production of O2 will be higher than its consumption, while during the night the photosynthesis stops, although the burning of glucose goes on.

Thus, during night, there is no production of O2. Only the consumption continues. The quantity of O2 is never continuously the same.

Dissolved oxygen is one of the most important factors in the pond. Many processes in the pond depend on the oxygen level in the water. It has also an impact on activity, growth and health of our Koi.

For example: You need 500 g of oxygen to metabolise 1 kg of Koi food (we won’t bother you with the formula).
The solubility of oxygen in water is related to the temperature and the atmospheric pressure. Ponds on a great height are under a different barometric value (mm Mercury). The higher the atmospherically pressure, the higher will be the solubility of nitrogen, argon and the other gases.

For the Koi keepers, it is very important to know that cold water contains more oxygen than warm water. In warmer water the fish need more oxygen. The metabolism is higher and the waste production is larger, whilst the bacteria in the filter need more oxygen for their transformation work.

Especially during warmer weather, insufficient oxygen in the pond occurs because of an insufficient water movement or aeration. At the same time, the abundance of underwater plants will lower the oxygen level too.

The depth of the Koi pond influences the oxygen level too, as does the quantity of organic waste in the pond and the amount of dissolved minerals in the water. If the pond is polluted, the saturation percentage will change completely. The waste needs a lot of oxygen to be transformed; therefore it is clear that you must keep your pond and filter aerated and as clean as possible.

If there is insufficient oxygen, the Koi will hang like balloons on the surface next to the water inlet, where the amount of O2 is the highest. They will breathe quickly and take gulps of air. If there is not enough oxygen in the pond, a lot of fish may die and the biggest ones first! You have to react very quickly by giving extra oxygen to the water. We all know the reports of massive fish losses in rivers after very hot weather…

If you treat the pond with salt (even with as little as 5 g of salt), the maximum O2 level will go down by 0.2 mg/l: the solubility of oxygen in the water goes down. The higher the temperature and the salt level, the lower the O2 level in the water will be.

CO2 is very soluble in water. It can be too high in a pond where there is a lack of aeration, too much fish and many under-water plants. The Koi will get into difficulties. If you add oxygen by aeration, the pH will go up, the air will expel the CO2 from the water.

When there is a shortage of O2 in the pond, we can help by adding more movement to the water like a waterfall, wind, air pumps and water inlets; also the Venturi’s produce extra air. You have to create fresh oxygen by changing the water and spraying cold water on the surface. Oxydators used in aquariums give pure oxygen to the water (they work with oxygenated water and a catalyst).

To give an idea: an air stone of 20 cm, in a depth of 1 meter, with an airflow of 20 l/min, releases 250 g of oxygen per day. The deeper the oxygen stones (or spin drifter) are situated, the more oxygen they will add into your pond, because little air bubbles will give more oxygen: there will be more surface in contact with the water. If we want the maximum of saturation, all processes must be optimal and fast.

The minimum oxygen level should be 7 mg/l to have a good functioning in the pond and filter. This is called the absolute oxygen level.

The relative oxygen level describes the percentage of the real oxygen saturation in relation to the maximum O2 saturation, which is 100 %. It tells us something about the quality of our pond system. 70 % can be enough in winter, but is far too less in summer; more than 100 % shows that you have too much algae in the pond … A saturation percentage of 80 % in a clean, well-aerated pond is within everybody’s reach.

There aren’t a lot of Koi keepers who measure the oxygen level in the water although it is a very important parameter for the health of the Koi. You can measure by using a digital O2 meter
Levels of oxygen in the water in mg/l

This table shows clearly what is said above. The upper row shows the water temperatures, the lower row the level of oxygen:

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 32 34
14,6 13,8 13,1 12,4 11,8 11,3 10,8 10,3 9,9 9,5 9,1 8,7 8,4 8,1 7,8 7,5 7,3


Over-aeration happens sometimes on hot days, when you have a lot of algae. You can see bubbles coming out of the “bottom “ to the surface.

Too much aeration can lead to the bubble-gas disease. With a high pressure in the water, the fish have more nitrogen and oxygen in the body. If the pressure suddenly goes down, the dissolved air in the water and in the fish wants to go back to the solution where it was before. But the amount of dissolved air in the fish can’t be exhaled that quickly and will form little blisters on the body. They can lead to the feared gas emboli, bulging eyes, bubbles in the fins and sudden death. Treatment: move the water very strongly, the gasses can disappear, but it can take weeks before the Koi is cured.

Conclusion: Our Koi need a lot of oxygen for their metabolism, you have to aerate the water especially during night, 7 days the week! Only when the water temperature goes below 15 °C, you can switch off the oxygen in your pond, but NEVER in the filter!



Ammonium NH4+ and ammonia NH3 are related; they both come from the respiration of the Koi and from the decomposing organic waste.

Ammonia is a very toxic and invisible gas. The value can never be 0 in the pond. There is always the breathing of the Koi which exhale ammonia… With the test kits you will measure ammonia and ammonium together, there is no obvious difference.

Ammonia consists of nitrogen N and hydrogen H.

Nitrogen is a constituent of all proteins, and proteins are found in all Koi food. Thus, by feeding the Koi, you add ammonia to the water! The aerobic cycle of nitrogen is one of the most important chemical processes in our pond.

Rising temperatures and pH values increase ammonia toxicity. A great part of the NH4+ will decompose into NH3.
The latter is many times more dangerous than ammonium ions.

This is important for the transport of the Koi, because the exhaled ammonia cannot escape anymore and the fish will get poisoned in the transportation bags… Therefore, the Japanese breeders lower the temperature of the water and prevent the fish from eating during a fortnight so that there will be less ammonium in the bags.

If the ammonia level rises too high, it causes irritation to the skin and gills. If the value is around 0.2 m/l, it will damage the skin, fins and gills. The gills will be corroded and there is less O2 intake. The fins will look like burned. The fish will leap out of the water…they will grow listless and are attacked by parasites, because their immune system is weak.

Peaks of ammonia occur in new ponds, when the filtration system is not ready yet, after the use of “products” and after big changes in the conditions, e.g., when you put more fish into the pond, if you give more food or if the food is too rich in proteins.

From a value of 0.3 mg/l, the toxicity goes up, eventually killing the little Koi first.

When there is an excessive level of ammonia, you must change 1/3 of the water daily until you reach acceptable values. You can use Zeolith too in the filter system, which will absorb the ammonia. You can’t use Zeolith as a permanent filter medium; use it only when you have ammonia peaks in your pond.

Go slow on the food; give a few days nothing and keep on measuring. The value has to be below 0.1 mg/l. Search the source of the problem!

The nitrosomas bacteria will transform the ammonia into nitrite, which is also toxic for the fish.



Nitrite is a toxic disintegration product; it is the step between ammonia and nitrate, formed by the nitrosomas bacteria.

The best value is 0, it shouldn’t be higher then 0.1 mg/l…

If you have high values, it is a sign of a failing filtration system. Those high levels prevent the fish to absorb oxygen and they will eventually die of asphyxia. Because of the lower level of oxygen in the blood, it will look “brown”.

The gills will be brown too and the fish hang on the water inlet.

If you have high values, stop feeding the Koi; water changes have little effect.

If you add salt to the water (3 g NaCl/l) the nitrite level will decrease. Take the Zeolith out of the filter before adding salt!

Ozone lowers the NO2 level too. It oxidizes nitrite to nitrate.

After an ammonia peak, there is often a nitrite peak.


Nitrate is the result of the transformation of nitrites by the nitrobacter bacteria, and it is normally not harmful to the fish. It is a primary nutrient for algae and plants. At high concentrations, the algae and plants will bloom while the Koi will loose their colour.

Nitrate will be more toxic when the pH is low, combined with hard water…

Nitrate is toxic from 300 mg/l onwards.

Change water if the value is too high (check the nitrate value in the tap water too!).


Cl2 is being used as a disinfectant for drinking water (between 0.5 and 2 mg/l).
In low concentrations, it is not harmful for humans, but can be highly toxic to Koi: it will damage their gills.

Chloride Cl- is without danger and is found in many combinations in salts.

The degree of toxicity of Chlorine depends on the temperature, pH, oxygen content of the water and the amount of chlorine added. There will be stress and discolouration of the fish from 0.01 mg/l onwards, and they will die if the value amounts to 1 mg/l.

Here in Spain the chlorine level increases in summer, so never add tap water directly to your pond, and always spray it first over the surface. Then, the chlorine will evaporate, or use a special product that neutralizes the chlorine (sold in specialized shops).

Conductivity: is measured in µS/cm; it is related to nitrate and salt in the water. It will rise if you used salt treatments. Values between 1000 and 1500µS/cm are not toxic to Koi.


PO4— is a non-toxic waste product in the water. When the value exceeds 1 mg/l, there will be more algae in the pond. You can lower the quantity with water changes, but if you use well water, the level of PO4— in your deposit water can be very high too!!!

Phosphate. Water Management for Koi Carps