I've been using CO2 injection on my 75 gallon tank for several months
now. I noticed today that every plant was just covered with bubbles
which normally would be a good thing except that it started happening
all at once. I also noticed that I'm getting a continuous stream of
bubbles from the Co2 tank. The gauge which was up at 900PSI is now
down to 500PSI. When I checked the water parameters, I have a Kh of 16
and pH of 6.6. Here are my questions:

1) Even though the pressure in the Co2 tank has only dropped 40%, I
seem to recall that the drop to 500 means it is almost empty. Is that
correct?
2) I have an Azoo Co2 Indicator which is showing green (OK), but
according to the charts, my Co2 is 115 PPM which is very high... are
the Azoo indicators worthless?
3) My tap water is natively about Kh 20 so I use exclusively untreated
RO water for water changes. My plan was to bring the Kh down to about
7 and then start mixing tap water in with the RO water. After six
months or so, I would have thought the kH would have been lower by now.
Any idea what's going on here?

Thanks