If there is one truth in koi keeping that every experienced hobbyist eventually learns, it is this: you are not keeping fish — you are keeping water. The quality of your pond water determines everything about your koi’s health, colour, growth, and longevity. Understanding what to test and what the numbers mean is the foundation of responsible koi care.

Ammonia: The Silent Killer

Ammonia is produced by fish waste, uneaten food, and decaying organic matter. It is the most dangerous substance in your pond because even low concentrations are toxic to koi. Ammonia burns gills, damages internal organs, and suppresses the immune system.

Your target ammonia reading should always be zero. Any detectable ammonia indicates a problem with your biological filtration — either the filter is undersized, has not matured, or has been disrupted. Common causes include overstocking, overfeeding, cleaning filter media with chlorinated water, or adding new fish without quarantine.

If you detect ammonia, reduce feeding immediately, add aeration, and investigate the cause. Do not simply add chemicals to neutralise it — address the root problem.

Nitrite: The Second Stage Toxin

Nitrite is produced when beneficial bacteria convert ammonia in the first stage of the nitrogen cycle. While less toxic than ammonia, nitrite is still dangerous — it binds to haemoglobin in koi blood, reducing their ability to carry oxygen. Fish affected by nitrite poisoning may gasp at the surface or appear lethargic.

Like ammonia, your target nitrite reading should be zero in a mature pond. Elevated nitrite usually indicates that the second stage of your biological filter — the nitrite-to-nitrate conversion — is not keeping pace. Adding salt at a rate of 1 to 2 grams per litre can help protect fish while you resolve the underlying filtration issue.

Nitrate: The End Product

Nitrate is the final product of the nitrogen cycle and is far less toxic than ammonia or nitrite. However, high nitrate levels over time can stress fish and promote algae growth. Aim to keep nitrate below 40 ppm through regular partial water changes — typically 10 to 20 percent per week.

pH: The Acid-Alkaline Balance

Koi thrive in a pH range of 7.0 to 8.5. More important than the exact number is stability — sudden pH swings are far more dangerous than a slightly high or low reading. A pH crash, where the water suddenly becomes acidic, can kill fish within hours.

Test your pH at the same time each day, as it naturally fluctuates — lower in the morning and higher in the afternoon due to plant and algae activity. If you notice your pH swinging by more than 0.5 units in a day, your buffering capacity needs attention.

KH: Your Safety Buffer

Carbonate Hardness (KH) is the most underappreciated parameter in koi keeping. KH acts as a buffer that prevents pH crashes. When KH drops too low — below 80 ppm — your pond loses its ability to resist pH changes, and a sudden crash becomes possible.

Maintain KH above 100 ppm, ideally between 120 and 180 ppm. If KH is low, add sodium bicarbonate (baking soda) gradually — approximately 50 grams per 1000 litres will raise KH by roughly 10 ppm.

Temperature: The Metabolic Driver

Water temperature controls your koi’s metabolism, immune function, appetite, and susceptibility to disease. In Gauteng, pond temperatures can swing dramatically between seasons and even between day and night.

Key temperature thresholds to remember: below 10 degrees Celsius, stop feeding entirely. Between 10 and 15 degrees, feed sparingly with wheatgerm food. Above 15 degrees, normal feeding can resume. Above 25 degrees, increase aeration as warmer water holds less dissolved oxygen.

Dissolved Oxygen: The Invisible Essential

Koi require a minimum of 6 mg per litre of dissolved oxygen, though 8 mg per litre or above is ideal. Oxygen levels drop when temperatures rise, when fish stocks are high, and when biological filtration is working hard. Ensure your pond has adequate aeration through air pumps, waterfalls, or venturi systems — especially during summer.

How Often Should You Test?

At minimum, test ammonia, nitrite, pH, and KH weekly. During spring start-up, after adding new fish, or during any health issues, test daily. Keep a log of your results so you can spot trends before they become problems.

A simple notebook or spreadsheet tracking date, temperature, pH, ammonia, nitrite, and any observations about fish behaviour can save you a great deal of trouble over the years.

When Water Quality Alone Is Not Enough

Perfect water parameters are essential but not sufficient. Parasites and bacterial infections can strike even in pristine water, especially during seasonal transitions. Regular professional health assessments complement your water testing routine and catch problems that water tests cannot detect.

Need help with your water quality? Contact KoiDoc for a comprehensive on-site consultation. Every visit includes full water parameter testing alongside microscopic health assessments for your fish.