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Regular testing of your koi pond water

by Chris Thomas January 01, 2024 10 min read

Regular testing of your koi pond water - Kitsu Koi

Spend enough time with koi keepers and you will hear the phrase ‘We don’t keep koi, we keep water’. Koi, and other ornamental fish obviously live in water and can’t function without it. Without quality water, they can’t function correctly either. 

If you spend more time with ‘online’ koi keepers, you will hear contradictory information when it comes to water testing and its importance. Unfortunately some advice is given against the importance of testing. 

Water quality has a direct impact on koi health and condition, so testing your pond water and maintaining a good environment can only be a good thing. 

In this guide we will cover what to test for, why you should make regular testing a habit, when to test, testing methods, interpreting results, and the benefit to your koi and pond environment. 

The guide is aimed at beginner to intermediate koi keepers - the information included is to provide koi keepers with the basic knowledge to test pond water for common parameters.

If you are interested in growing jumbo koi, or raising koi at a competition level, then you may wish to expand test parameters, and even manipulate water parameters beyond temporary pH adjustments. We will produce an advanced guide at a later date, covering the methods and benefits to controlling pH.

What should you be testing for in a koi pond?

The minimum, basic water tests for keeping koi should be Ammonia, Nitrite, pH, KH, and Chlorine. Keeping Ammonia, Nitrite, and Chlorine at undetectable levels and a balanced ph and KH will promote a healthy environment for koi to thrive in. We cover the importance of KH in our blog post, What is KH and why is it important?.

Why should you test your pond water regularly? 

People will argue that they have never tested their pond water, but ignorance is bliss. Water quality can change rapidly, leading to poor fish appearance, health, or at worst case, mortalities. 

Regular water testing, even when things are looking good, can predict or highlight recent water chemistry changes before the fish are affected by poor water quality. Poor water quality can be caused by equipment failure, poor performance of dechlorinators, change in source water quality, over feeding, and other factors that affect the filter performance or biological load. 

Once a pond has matured, the Ammonia and Nitrite should remain stable so long as no significant changes occur. Although Ammonia and Nitrite should be stable, they are toxic to koi and goldfish, and will cause poor immunity at low levels and death at elevated levels. Testing for Ammonia and Nitrite each week will highlight any trend before it’s an issue. When adding new fish, or after treatments, or any major event that may influence bio load, increase the frequency of your water testing. 

Constant demand for carbonates will deplete KH over time depending on your source water, so regular testing KH and pH of the pond water and the tap water will help maintain a long term balance. Carbonates are required by the filter bacteria to process ammonia, and are used up by any acids introduced to the pond (such as carbon dioxide from fish respiration) so depletion of carbonates can happen faster in highly stocked ponds. The KH level influences the pH level of a pond and the koi thrive in a stable environment so we want to maintain a steady level of pH and KH. 

When changing water or topping up after cleaning our filters, we should make sure that we are not adding chlorine or chloramine from the tap water. If you use a carbon based dechlorinator, then you should test the water in the pond weekly, and test the water AFTER the dechlorinator periodically - the time period will be different depending on the size of your dechlorinator, amount of water used, and the quality of the source water. We would recommend testing the tap water at the same time to make sure the chlorine or chloramine levels have not changed. 

You should also test your pond water if the behaviour or appearance of your koi changes. If your koi are gasping at the surface, flicking, flashing, jumping, holding fins close to the body, has red skin, excess mucus, bulging eyes, fin damage, or jumping, then water quality is one possible cause. Understanding what is causing this behaviour or damage is important as it can lead to further damage. Knowing your water results will guide you to the correct course of action. 

If you are investigating a koi health problem, and you find parasites, then you will need to use a treatment in your koi pond. Some treatments are toxic in different water conditions, for example Chloramine T dose rates must be adjusted for pH, and Colombo Alparex requires a KH of 6dKH for safe use. Please test before using chemicals in your koi pond. As well as safety, some treatments are more effective at different pH levels, so that may influence your choice of treatments.

How to test your pond water

The most simple and most frequently used testing method is to add a reagent or set of reagents to a sample of pond (or source/tap water). These are commonly referred to as a liquid drop test. A reagent is a substance or mixture for use in chemical analysis or other reactions. The reagent will react with the target chemicals (such as Ammonia or Chlorine), changing the sample to a specific colour which is then compared to a colour chart supplied by the manufacturer. Some manufacturers will offer a kit containing most, if not all the required test kits. 

 Colombo Test Lab


Colombo Test Lab - Multi Test Kit Tests For Ammonia (NH3), pH (acidity), KH (carbonate hardness), GH (general hardness), NO2 (nitrtite), NO3 (nitrate) and PO4 (phosphate). The test kit also includes test tubes, colour charts, syringe and a manual instruction booklet.


Different water parameters may have a few different testing methods, for example for testing ammonia there are two methods, so colour charts may not be compatible between brands. It’s important to use the correct method when viewing the sample. We often see posts on social media where the hobbyist shows a picture of a reacted sample, but the colour chart is not being read correctly. When you compare the reacted sample to the colour chart, it is very important to look at the correct angle, with the correct lighting - as an example the Salifert Nitrate test used by marine keepers changes the value of the readings by 5x depending if you look from the side or the top.

The correct way to view the Colombo pH Test Kit The incorrect way to view the Colombo pH Test Kit


When using the Colombo pH Test kit, you should check the sample from above as per the first image. The colour charts have a brilliant cut out, that perfectly fits the supplied test vials. In the second image, the colour of the sample looks different, due to the amount of light passing through. This will cause you to read the incorrect result. It could make a big difference if you're using the incorrect pH level to make decisions on pond maintenance or treatments.


Electronic devices can be used to provide more accurate results, or even constant monitoring of the vital parameters. The most popular way to accurately measure Ammonia, Nitrite, pH, KH, and Chlorine using an electronic kit is to use the Hanna Pocket Checkers. Hanna Pocket Checkers use a regent to measure the specific test parameters, by shining a light through a sample of reacted pond/source water. These are called photometers. A photometer takes all the guesswork out of matching the colour of a reacted sample with a colour chart, and gives a digital readout. The accuracy and resolution of the test is impressive. A Hanna Pocket checker for Ammonia can display a reading of 0.01mg/l, versus a Colombo Ammonia test, where 0.25mg/l is the first step on the colour chart. 



Whether you use a liquid drop test kit, or an electronic device, you should have a few syringes for taking accurate measurements of sample water. Drop test kits often have a reaction time that needs to be measured before taking your reading. Most people have a phone with a stopwatch, but if you’re not reading this on a smartphone, get a stopwatch for your testing kit. 

Constant monitoring of pond parameters can be especially useful for people who work long hours or are away from home for long periods. Not all parameters can be constantly monitored at reasonable cost to the hobbyist, but if you wanted to track Ammonia and pH, then Seneye offer a range of products to make this possible. Hanna and BlueLab offer pH monitors, especially useful for people with a lot of fish, or tap water supplies with a low level of carbonates. 

Seneye Pond pack with wifi, float, and weatherproof box Hanna ph monitor with TDS probe BlueLab Guardian - pH and TDS monitor for koi ponds
Seneye Pond  Hanna pH Monitor BlueLab Guardian


Whichever method you use to test your pond water, maintenance of your equipment is essential. Liquid drop test kits and photometers will use a test vial of some sorts, and this needs to be kept clean and free from contaminants. Ensure your test kits are in date. Once reagents are open to the air, they will slowly react so although  a test kit can be in date, if you used it months ago, it's been deteriorating. If you use any constant monitoring, probes need to be calibrated and cleaned. 

Especially for new koi keepers, it would be wise to use a notebook, spreadsheet, or an app to record your pond parameters. This will allow you to evaluate how your pond filter is maturing, or help you see trends in water quality and source water quality. Seeing a trend will influence how you maintain your pond, and can prevent a negative environment. 

Seneye has an amazing web portal for its users, with graphs tracking Ammonia, pH, and temperature, which is recorded every 30 minutes. The added benefit of a Seneye is that it will not only test the ammonia and pH, but by monitoring temperature, you may discover a fault with your pond heating system, if you're heated. As a secondary benefit, if the Seneye loses power, the app will notify you so it can act as a warning if you have a power outage.

Interpreting test results

When you have your test results, you then have to figure out what it means and how it affects the world your koi live in. 

Ammonia and Nitrite - These should be undetectable. On a drop test, that means 0mg/l. If you’re using something like the Hanna HI-700 or HI-711, then I have included tables below. Due to the resolution of photometers, a true 0.00mg/l is unreasonable to expect. Ammonia toxicity is affected by temperature and pH, but test kit’s do not account for this relationship. Simply, Free Ammonia is the toxic form and is present at high temperatures and high pH. Ammonia is found as non toxic Ammonium at low pH and low temperatures. However, as pH and temperature can vary, we would assume the reading is the toxic, Free Ammonia. A 10-20% water change is a very quick, basic solution to an elevated ammonia or nitrite level. 

pH - Koi will live in a pH range between 6.5 - 9.0, but stability is more important than a specific number. The pH of a koi pond will influence the toxicity of chemicals, and the performance of the bio filter. Keeping the pond above a pH of 7.0 will benefit biological activity and the filter performance. If you pH is out of range, do not try and change the level rapidly as this could cause further stress, start with small water changes. If the levels are dangerously high, then Colombo Morenicol pH Min can be used, or if you’re experiencing a pH crash (less than pH7), then Colombo KH+ will raise KH to raise and stabilise the pH. 

KH - The ideal KH will vary between hobbyists, but for a stable pH and effective bio filter, aim to keep your KH in the 4-8dKH range. If you test your tap water, and it’s below 4dKH, then you may struggle to replenish enough carbonates from water changes alone. A good level of carbonates will promote a stable pH, reducing stress in koi and supporting a healthy bio filter.

Chlorine - The maximum recommended amount of Chlorine is 0.02mg/l, and 0.00mg/l would be the target level. You should use a Total Chlorine test that will include Free Chlorine and Chloramines. To get an accurate result, you should use the Hanna HI-761 Ultra Low Range Total Chlorine Checker, which will give accurate results at a 0.01mg/l resolution - perfect for koi keepers. If Chlorine is found in the pond, a quick solution is to add a liquid dechlorinator, or Pond Prime. When you check the supply after your carbon based dechlorinator and find chlorine, the first step would be to assess the flow rate. The water must be in contact with the carbon for a sufficient time to be adsorbed (that’s not a typo), and that time will vary depending on the concentration of chlorine/chloramine in your tap water and the amount of carbon you have. Over time, the carbon settles and water creates paths, so if the flow rate is correct, then back flushing the carbon should restore the ability to create chlorine free water. 

Be aware that some test kits can be affected by chemicals in the pond. Formalin and Malachite will interfere with an ammonia test for example. When using things like Seachem Prime, that binds ammonia, you will still get a reading for ammonia. 

The benefits of regular water testing

By understanding your water chemistry, it can improve the enjoyment of your hobby. Although immediately you see a small financial and time cost to regular testing, it’s a worthwhile investment. 

Monitoring the pond environment can help you plan and react to changing conditions, preventing you wasting money on ineffective treatments, reducing damage to koi, and keeping your prized koi in top condition. Knowing the water quality and the limits of your system can help you achieve better results in growth, and stop you wasting food if water quality isn’t good for growth. Having control of water quality increases the ability of the koi to fight parasites, so can cut down on reliance on chemicals and treatments - saving you money, reducing stress for you and your koi, and avoiding the side effects of chemicals in a koi pond. 

Spending time by the pond while you test your water gives you time to observe your koi, whether that’s watching their behaviour, assessing their growth and development, or just enjoying your beautiful pond.

Kitsu Koi - Taking a coffee break by the main pond

If you have any further questions on the above, feel free to call in. We can chat over a coffee, and can even demonstrate the correct use of the test kits, or you can bring a sample of your water and use our Hanna Photometer. We stock a range of test kits, reagents, dechlorinators, filter starts, KH buffers, and pH adjusters. 

Chris Thomas
Chris Thomas

Chris has been keeping koi for over 25 years, and dealing in koi for 15 years. Travelling to Japan to select new stocks is the favourite aspect of the business, closely followed by being on the tools pond building.

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