There are many Koi water quality hazards. The biggies are ammonia and nitrites and, to a lesser extent, nitrates. Some others that are important to have knowledge about are chlorine, chloramines, pH balance and random contaminants.
Both chlorine and chloramines can harm fish and can burn or kill pond plants. They will also kill the beneficial nitrifying bacteria in your Koi pond.
Chlorine, which is a volatile gas, breaks down with water circulation and exposure to air within 24-48 hours. Chloramines, on the other hand, take much longer to dissipate.
Municipal water supplies have begun adding ammonia along with the chlorine which results in the longer-lasting chloramines. If you add water to your pond from a municipal supply make sure to spray it in with a hose to add aeration that will help break down and move out the gases.
Another good option would be to let the water sit for a day or two before adding it to your pond to make sure it doesn't constitute a Koi water quality hazard!
Sodium Thiosulfate removes chlorine from water and will also pull chlorine from the chloramines. You can make a stock solution by adding four ounces of Sodium Thioslufate crystals to one gallon of distilled water. One drop per gallon (50ml per 1,000 gallons) will safely de-chlorinate your pond.
pH is a measurement of whether the water is acidic or alkaline. pH is measured on a scale ranging from 1 to 14 with 7.0 being neutral
A pH range of 6 to 8.5 is considered to be acceptable for most pond life. The biggest worry associated with pH is its direct relationship to the toxicity of ammonia and nitrite. For each pH number above the neutral 7 there is a tenfold increase in those toxicities.
When the pH of your pond drops below 7 it is considered acidic. To raise the pH try adding baking soda or ground limestone.
If your pH exceeds 8.5 your Koi's immune systems will be stressed to the point of becoming ill. A common cause of this can be cement or mortar leeching lime into your pond.
You do need to watch out for a "pH crash". A "pH crash" is when in a relatively short period of time the pH begins to drop and won't stop until it reaches 5.5. This can happen literally overnight in a small pond. At a pH of 5.5 Koi will begin dying within a few days.
Any time you see all the fish in your pond start to act differently at the same time you should suspect a pH crash. If this occurs you can raise the pH by adding 1 cup of baking soda for each 1,000 gallons of water and check and repeat every two hours until the pH is back to at least 7.0.
Run-off water is the main way that contaminants affecting Koi water quality enter your pond. Run-off water from a nearby stream, or collected rainwater may contain toxic insecticides, herbicides and/or fertilizers.
Rainwater from metal roofs or asbestos shingles will contaminate the pond and can prove toxic to both your Koi and your plants.
You will also want to be careful with your lawn care practices. Don't use anything on your lawn or garden that you don't want in your pond! Particularly if your pond is on lower level ground.
If you see white foam near your waterfall it can indicate a high level of dissolved organic compounds and you should perform some partial water changes and perhaps use an additive to help handle the higher organic load.
As you can see, you need to carefully monitor your pond water. If you are educated about what to test for, when to test and signs of some of the more common problems, you should be well equipped to maintain a high level of Koi water quality!