Pond Water Quality

In general, everything you should know about your pond water is available in some sort of simple pond test kit. Most kits are very easy to run. Run them at least weekly during the first part of pond opening but monthly or more during the season.

If fish ever get sick or start acting funny, you should know that SEVENTY PERCENT of fish health problems, INCLUDING vulnerability to disease-causing organisms – are related to WATER QUALITY.

Important: DO NOT make the mistake of not testing for pH and ammonia at the first sign of trouble.


Ammonia is the primary waste product of fish, excreted primarily through the gill tissue, but to a lesser extent via the kidney. Ammonia can also accumulate from the decay of fish tissues, food and other organic debris derived from protein. Ammonia accumulations cause reddening of the skin and disability of the gills by its direct caustic effect on these surfaces. Fish suffering in water with high ammonia accumulations will isolate themselves, lie on the bottom, clamp their fins, secrete excess slime, and are much more susceptible to parasitic and bacterial infection. Ammonia is a big problem in new systems because the bacteria that would naturally dissolve ammonia are not established, see discussion of cycle. As well, even in established systems, ammonia may accumulate in springtime when the water is cold but fish are eating, because filter bacteria have not emerged usefully from hibernation. Ammonia is capable of ionization below pH 7.4 and so in its ionized state is less toxic to fish. Above pH 8.0 most ammonia is ionized, and so becomes more toxic. Care should be taken not to increase the pH of a system if ammonia is present but the need to drop the pH or restrict oxygenation to tanks of fish to keep pH down is an overrated aberration in the literature. Treatment: Water changes and management of the pH near neutral will go a long way to cutting losses from Ammonias, ancillary, less useful modes of Ammonia management include the use of the various water conditioners that bind ammonia, and the application of rechargeable Zeolites to the system filter. Water that is warm, high in pH or deprived of oxygen will have an enhanced toxicity when ammonias are accumulating. These are all important considerations as we try to interpret the varying symptomatology of fish at the same ammonia level, for example, but are affected very differently.


Nitrite is an intermediate metabolite in the CYCLE. Nitrite binds fish Red Blood Cells causing gasping and brown blood disease. Fish that die with their gills widely flared died of Nitrite induced “met-hemoglobin-emia” Ways to combat nitrite intoxication are firstly to identify the problem, usually a simple test will tell. Secondly, performing partial water changes will lower Nitrite levels. Thirdly, addition of salt will usually inhibit uptake of Nitrites. Fourthly, in many instances, cleaning or upgrading the filter will correct the problem. And finally, the feeding diet should be suspended or reduced during a specified amount of time to counter-attack the nitrite intoxication.


Nitrates are a natural by-product of the bacterial “reduction” or removal of Ammonia and Nitrite in the natural pond’s ecosystem. Nitrates are an under-estimated fish killer. When fish are sick, and the history contains some information to suggest the pond has been set up for a while, you can bet Nitrate levels are part of the problem. This is especially true for Goldfish and “flipover” disease. Once dismissed as a “non” threat to Koi and Goldfish, exceptional informatioin exists to suggest this is not true. Scientists initially evaluating Nitrates as a non-toxin did not test their subjects long enough. Nitrate accumulations cause dilation of the veins in the fins and other health problems. Never let your nitrate levels exceed 100 ppm or illness and vulnerability to disease will be the result!

Carbon Dioxide

CO2 can exist in water independently of dissolved oxygen. If the level gets too high, even with normal dissolved oxygen levels, illness will result. Many atmospheric gases dissolve in water to some extent and carbon dioxide is one of the most important. It dissolves readily to form carbonic acid, giving a weakly acidic solution. This chemical reaction has important consequences, not only for the eventual chemical content of water, but also for other chemical and biological reactions that occur in the aquatic environment. The naturally acidic nature of rain allows it to bring other, less reactive, substances into solution. (The pH of naturally formed rainwater is about 5.6 but, as we shall see, local atmospheric and landscape variations can alter it significantly before it gets anywhere near our fish ponds.) The affinity between carbon dioxide and water has many far-reaching effects on water quality, playing a major role in plant and animal respiration and pH buffering.

Dissolved Oxygen

Hypoxia is ‘oxygen starvation’. Most koi ponds are usually well served with venturi retums or waterfalls, but poor pond maintenance, high stocking levels and unusual climatic conditions can lead to low dissolved oxygen (DO) levels. Low DO is likely to occur in summer. As water becomes warmer it can progressively hold less oxygen: and the fish become more active, leading to a greater demand for oxygen; and the bacteria in the pond and filters need more to, as do submerged green plants including algae. The role of submerged plants and algae should perhaps be clarified. During photosynthesis, submerged plants release oxygen into the water, which is why they are often called oxygenating plants. However, they also respire at the same time, extracting oxygen from the water and excreting carbon dioxide. During daytime they produce more oxygen than they consume, but at night, when photosynthesis ceases, respiration continues and they become net oxygen consumers. Clearly, if the oxygen demand exceeds the oxygen supply then the DO levels will gradually decline and this presents a serious danger to the koi. Common causes of low DO, apart from high fish densities, are heavy feeding and a dirty pond or filter. A lot of oxygen can be used in oxidising organic waste and, under certain conditions, this extra demand may be ‘the straw that breaks the camel’s back’. Unfortunately, a DO problem often occurs in the early hours of the morning. when we are not there to see its direct effects on the fish, rather than during the day when submerged plants are releasing oxygen from photosynthesis.