Basic water testing can tell you many things about your pond, but many things can not be practically tested for. Typical pond testing will not show the presence of most chemical or organic poisons, nor will it show the presence of disease organisms.
Usually pond water is tested for pH (a measure of the acidity or alkalinity of the water), ammonia level, nitrite level, nitrate level, phosphate level and salt level. Additionally, hardness testing, both general and carbonate, may be informative and for some problems copper or iron testing may be indicated. All of these tests are readily available to the pond hobbyist. More advanced testing or testing for poisons or disease organisms are usually done by a specialty lab.
We use and recommend liquid drop type test kits rather than test strips. Test strips, particularly multitest ones, are very difficult to distinguish fine gradations with and only have about a 12 month shelf life, frequently making them out of date shortly after or even sometimes before you buy them.
What do these tests tell you? Let’s start with pH. pH is a measure of the acidity or alkalinity of your water. The scale ranges from 0 to 14, with 7.0 being neutral. Koi will thrive with the water anywhere between 6.5 and 8.0 and if your water is in that range you need to do nothing to it. Below 6.0 your fish are in immediate danger and a pH increasing chemical should be added or a major water change begun. Above 9.0 your plants will usually start to suffer first, but your koi will eventually also be affected. Many commercial products are available to adjust your pH up or down, with various effectiveness. It is best to use a buffer to help stabilize the pH after you have it in the correct range. A buffer helps keep the pH from fluctuating up or down.
Ammonia and nitrite are both toxic byproducts of the breakdown of organic wastes in the pond. Additionally, ammonia is exuded directly from the fish’s gills. In any new set-up or any time the biological filter is cleaned and restarted the ammonia level will climb for the first 14 days or so until the bacteria in the filter catch up on processing it. After the ammonia spikes, the next week to ten days will see an increase in nitrite until it spikes and starts back down. There are commercial products to help lower both of these until they level out. In a mature pond they should both read zero to negligible.
Nitrate is the final stage of the pond’s biological filtration cycle. High nitrate levels are not usually toxic to your fish unless extreme. Nitrate is a plant fertilizer and sufficient plants will remove the nitrate. High levels of nitrate and phosphate will feed algae blooms and without enough plants, water changes may need to be done to keep these levels under control.
Salt is used to help with the fish health. At levels of 0.10 to 0.14 percent it will not hurt plants and at levels of 0.20 to 0.30 is therapeutic in non planted ponds. Levels of 0.40 to 0.50 are suitable for hospital / quarantine tanks. Liquid drop test kits are no longer available to the hobbyist. Digital salt testers are available but expensive. Salt water aquarium salt level testers don’t read finely enough for the small levels used in ponds. This leaves you with bringing a sample to someone to be tested, but, as salt does not evaporate out and is only lost when you have an overflow, leak or do a water change, it is sufficient to test your salt level two or three times a year.
Chlorine / chloramines testing should not be necessary as chloramines remover should be used any time tap water is added to a pond. For practical purposes you can not overdose on dechlorinator, as even 100 times the recommended dose will not hurt your fish or plants.
In review, we recommend testing your pH at least monthly. Test your ammonia monthly unless you have a new set-up or have cleaned your filter. If either of those is the case, test both ammonia and nitrite daily until they level out at close to zero and chemically adjust as needed. Test your salt, phosphate and nitrate about once every four months.