pH and pH Crash
Written by Dr Erik Johnson
PH: When it Falls, So Do Your Fish. What Supports PH?

PH is only a measurement of the free Hydrogen ions in the water and is very, very dynamic. The pH 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 can change overnight.

In simple terms, the pH is supported in range by the carbonate activity of the water (measured in Total Alkalinity) and when the carbonates are exhausted, or organics-overloaded, the pH plummets to 5.5 and kills filter bacteria and fish. In other situations, the pH can glide downward very slowly over months and thus your resident fish can survive a low-ish pH but the new fish you keep buying perish quickly as they enter the 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 as well. All these are simple measurements with cheap test kits.

If your pH tends to run low, like; under 7.0 all you need to do is add moderate amounts of carbonate or ph buffer, and check an hours after each application to make sure the carbonate alkalinity as at least 30-40 (that's AT LEAST)

In desert limstone areas, low pH is not the problem. Mike Anderson calls it liquid limestone, and the pH in those areas is VERY high. Carbonate alkalinity over 180 isn't nice for Koi. Causes a lot of shimi (black freckles)