Classic algal bloom sign...Thank you both. I've NEVER had high PH. Actually, I've frequently aerated to keep it from dropping too low. Was shocked to see it off the chart high when I got home.
Algae are plants.
Plants breathe carbon dioxide. Algae are no different.
What actually happens is the algae starts and takes up all the freely available carbon dioxide to live on. Fairly quickly this is exhausted so it starts to break the bicarbonate ion into carbon dioxide gas and the outcome of this reaction is hydroxide which puts upwards pressure on the pH. Significant upwards pressure.
There is several other phases of an algal bloom and some of the chemical reactions are astounding, in some cases you can see this in small black flecks on the algal mass the small black flecks are carbon.
This is why pH 'off the charts' in cloudy water is a strong indication of algal bloom in progress. A typical algal bloom result might look like:
This seems illogical as low TA and high pH don't go together. This is in fact not correct. The TA method measures only the bicarbonate and some other minor chemical species. At pH 8.1 carbonate is the predominant species with a strong follow on from hydroxide and consequently not accurately reflected in the test results.