O2 does not lower pH, CO2 does that.
More than liely, and entirely a guess... you had way too much cyano in your tank, and killing it released an over-abundance of lethal bacteria. Remember, dino and cyano are poisonous.
This was my first thought when I read the post about the air stone lowering the pH...impossible unless you were pumping total CO2 into the tank via the air stone...the only way for this to happen is if the air the air pump was pumping into the tank had very high levels of CO2, this air comes from within the house...if this was the case no human could survive in that atmosphere. Yes, in the winter the CO2 levels will rise some in a house because it is shut up, but our houses still breathe during the winter, they are not air tight. If the CO2 in the house got that high to affect the tank via the air stone, everyone in the house including the animals would be dead. Increasing the CO2 makes carbonic acid which is what lowers the pH, thus increasing the air in the tank water, which is still going to have roughly 18% oxygen versus the 5% CO2, and 70+% nitrogen via the air stone is not going to decrease the pH.
With all of the being said, something else lowered the pH....my guess, the slime remover and/or the toxins released d/t the die off of the bacteria.
It is still a mystery to me as to why the fish died and the others are having issues. I have very strong feelings it has
something to do with the slime remover and is a direct result of adding that to your tank.
I still believe the red slime is part of a tank cycling and if it continues...then there is something off in the tank. If I had to make an educated guess...the cause has something to do with you water coming out of a well....especially from over in the Mahomet area, that water is usually loaded with additional minerals. What is also interesting to me...you and FastRC are both having major issues with the red slime algae...you both get your water from residential wells. This is a common factor in both of these cases and you guys are not located that far from each other...thus the content of the water in the wells may be very similar.
If it were me, in both cases I'd look at getting RO water from someone else for a while and use it for your top off and water changes and see if this happens to help at all. You could purchase it from a LFS if you wanted, but that would get awfully expensive. I'm sure there are quite a few of us here who could help you out with water. I know personally, most times I have well over 30 gallons of RO/DI water on hand and can make up 50 gallons of it in less than 8 hours. But like I said, this is what I believe is most likely the cause and where I would start until it is ruled out.