Low pH would not kill your plants unless you dumped some chemical into the tanks that was a plant killer. Dumping hydrochloric acid into the tank could do that if not done very carefully and slowly. Is it possible that some other member of the household dumped something into the water?
I believe my ph is starting to crash. I added diy Co 2. Before had ph 7.2 and Kh1. Now ph is to low to read. I added 1mm of baking soda and 2 mm calcium sulfate. Only the fish I bought the day before died.
No I don't have an air stone. Have diy Co2 similar to this:
Tube that connects to the powerhead is smaller, thus I was unable to get an airs tone in it.
I don't see the need for air stone for many with planted tanks don't have air stones or filters, thus I am confused about your statement for the need for an air stone.
I had been told by experienced aquariust that I can use diy Co2 even though my Kh is 1 but I think my tank may be the exception. For a few have red gills are breathing strangely. In fact 1 was upside down for a while. So I turned the diy Co2 off and add 1mm of baking soda.