pH is only a measurement of the free Hydrogen ions in the water and is very, very dynamic. The pH in your koi pond can change overnight…In particular, the pH is prone to fall, and “crashes” are quick, and fatal. Interestingly, many people say they checked the pH last week, and are surprised to find that the pH in their pond water can change overnight.
In simple terms, the pH in your koi pond is supported in range by the carbonate activity of your pond water (measured in Total Alkalinity “TA” and Carbonate Hardness “KH”) and when the carbonates are exhausted, or overloaded, the pH plummets to 5.5 and kills pond filter bacteria and your koi. In other situations, the pH can glide downward very slowly over months and thus your resident koi can survive a low pH but the new koi you keep buying perish quickly as they enter your pond water with a very acid pH.
Check the pH regularly, and also, to get a grip on your ‘safety net’, measure the water’s carbonate activity as Total Alkalinity (TA) and Carbonate Hardness (KH) as well.
If your KH reading is low, below 125, “Buff-it-Up”:http://www.aquameds.com/water-conditioners/buff-it-up/#.VLlzhUfF_P0 will help.