Water quality is the single most important part of a healthy koi pond environment. And tap water is the single worst thing a pond owner could ever add to their pond. Why? Because today's tap water is full of disinfectants like chlorine and chloramines, as well as heavy metals and trace pharmaceuticals, that are very toxic to pond life.
Water conditions can be determined by testing for pH, ammonia, nitrite, and salt. Another preventative measure is to follow routine pond maintenance schedules for each season because they include the addition of beneficial bacteria.