I understand what you are saying with regards to pH and have come to believe that conductivity, or total dissolved solids which may incude nutrients,minerals,salts,or organic matter are largely responsible for stress and or osmoregulatory problems when sudden changes occur. Is why we acclimate fish. Am researching problems associated with the transferring or acclimating of tropical fish to and from waters that may be high in conductivity or lower than what the fish came from. Although there are tests for such things as conductivity and or TDS, It would appear that most hobbyists,or perhaps I should say many,, rely on pH tests to indicate to some degree,the possible alkaline or acidic conditions that normally (not always) are associated to a particular reading from said pH test result.obviously one can have a pH value of say,7.8 and still have little buffering capacity but most of the time the pH value can be connected to in the case of 7.8 pH ,,a perhaps hard alkaline source.
My research thus far is incomplete as I am still learning or absorbing. Presently also studying blood acidosis ,and alkalosis and the effects over the longterm on tropical fish and the causes which seem to point to sudden changes in water chemisrty from acidic conditions to more alkaline conditions.
I appreciate your taking time to reply to my querys and I shall I think ,,try the high tech Co2 injected tank in the spring when plant selection is of much better quality. I fear I will need to do some further studying before I attempt to place fish in an enviornment where the daily shifts you speak of and their effect are still largely uncertain to me.