This has been a common issue for decades and is due to high indoor room air CO2. When CO2 enters water the pH drops.....period. It could care less what your Alk is. One can easily prove this to themselves if there is any doubt. Get a glass of water and put in some pH indic
ator and note the color. Now take a straw, put it in the glass and blown in it and watch before your eyes as the color of the pH indicator changes like magic to a lower pH color. Of course one can use a pH meter also. And go ahead and measure the Alk if you want, it will help zero and will be the same no matter what the low pH was in the glass.
So, how does this take place in a room. When the room is tight and CO2 can not escape the CO2 pressure in the room builds up as it can not leave. Our tanks also have what we will call CO2 pressure in the water. If the room CO2 pressure is greater than the water the CO2 goes into the water. It is like the room is a balloon and you open the ballon and all the air leaks out. Why, because the air pressure in the ballon is higher than the room. But all of the air in that balloon does not leave, only that amount leaves until the ballon pressure and room pressure are equal. Same for a tank. So, now we open a window and what happens ? Now most of the CO2 in the room escapes outside. The room pressure is now low but the tank pressure is still high. The tanks is now like the filled balloon and it now leavess the tank, out into the room and outside and the tank pH goes back up. With that glass test above with that now lower pH we can do the same. Put an airstone in it and the pH color will go back to where it was when you started before you blew into it.
These pressures have names pCO2 g (= gas in room or air), pCO2 aq (aq= in water). The p = partial pressure. Most tanks have a pH of 8.2 when equalibrated with the outside air. If it is lower than this you have CO2 issues and if it is higher you have added to much buffer, have to much algae or a dirty pH probe with algae on it (which raise the pH at the probe algae interface).
I had my windows wide open and my ph dropped to 7.6 the day of my gathering.
This can vary for many reason. The window may have been open but the CO2 was not leaving at a very high rate for what ever reason but the pH still dropped for that reason. Now if you had a bunch of fans around the house, blowing air out the window, this would not have happen.
In tight houses the indoor CO2 can be 2x that of the outside. The more modern houses with HVAC systems do not have this pH issue usually. Old houses like mine leak so much there is no pH issue with indoor CO2. Pretty bad when you get "indoor snow" :lol: Or your PC under a window is like it being run by a chiller.
