One of the most important aspects of successful koi keeping or any fish keeping for that matter, is biological filtration and its function in the nitrogen cycle. I read recently, that the number one reason novice fish keepers become disillusioned with the hobby is the frequency in which they experience high death rates of their aquatic pets after setting up a new system. This data applies to all types of fish but nonetheless, they're pretty staggering statistics. My very first four koi died in less than 24 hours. For some unknown reason to me at the time, they didn't like all that chlorine I had in the water to keep it clear.
Nitrite poisoning inhibits the uptake of oxygen by red blood cells.
Known as brown blood disease, the hemoglobin in red blood cells is converted to methemoglobin. This problem is much more severe in fresh water fish than in other marine organisms and can easily cause death. Nitrifying bacteria are classified as obligate chemolithotrophs. This simply means that they must use inorganic salts as an energy source and generally cannot utilize organic materials. They secrete a sticky slime matrix, which they use to attach themselves.