This is a composite of a series of articles dealing with the chemical makeup of pond water. How to measure what is in it, what is good, what is bad, and what to do about it. Before starting, I would like to discuss the name applied to what has typically been called the "bio-filter." A filter is defined as a porous device through which water (or gas) is passed to separate out matter in suspension. The biologic activity within the pond "filter" does not trap the matter in suspension but acts on dissolved components that could not be separated regardless of how fine the filter pores. Although this device may perform a dual role as a mechanical filter, to emphasize the processes of interest, you will see that I will often refer to it as the biologic converter or bio-converter, not as a filter.
By introducing fish into your pond, you have assumed the responsibility for the care of these creatures. This includes not only feeding them but also providing them with a healthy environment in which they can live and thrive. Partial determination of the quality of this liquid environment can be made through chemical measurements. It seems somewhat ludicrous that someone would spend hundreds or thousands of dollars to build a pond and then add hundreds or thousands of dollars of beautiful Koi but would not buy and learn how to use a ten dollar nitrite test kit. This doesn't mean that one must test the water every few minutes or even every few days. An established pond with the fish appearing healthy should be checked every month or so. It is only when something out of the ordinary is observed and possibly during seasonal changes when an additional test or two might be needed. A simple test at the right time may prevent a small problem from becoming a catastrophe. When starting up a new pond or bio-converter system, daily tests may be required until the converter comes on line, then weekly for a couple of months until the system has stabilized.
Just as when medicinally treating your pond, it is imperative to know the total amount of water in your pond and converter/filter system as accurately as possible. Over treatment or under treatment with chemicals can be equally disastrous. Don't guess on quantities, measure them!
Do not confuse the terms water quality and water clarity. Crystal clear water can contain compounds that are deadly to your fish. Green water, sometimes called an algae bloom and caused by excessive phyto plankton growth, can actually be beneficial to the fish although not very beneficial to the pond keeper who can't see them. Water clarity can give some indications as to mechanical filtration effectiveness.
A pond with a biologic converter and filled with Koi is a rather complex, somewhat self-contained ecological system. Each component of this system requires the other components to survive and prosper. The basic portion of the cycle is shown below. Fish waste and other organic waste is converted by bacteria and fungi to ammonia compounds. These compounds can be injurious to the fish, but a healthy biologic converter populated with families of bacteria consume these ammonia compounds and convert them to nitrite. Unfortunately, nitrite is just as toxic to the fish as the ammonia. Again, the biologic converter comes into play with another colony of bacteria that convert the nitrite to nitrate. The nitrate is basically inert to the fish but usable by plants and algae within the pond. As the plants and algae grow and the Koi eat them, the cycle starts all over again. The bacterial colonies that do this conversion are called aerobic since they require oxygen to convert their "food" to energy just like the fish.