Problem determining co2 from KH and PH readings
I've been using a drop checker and KH and PH readings to determine co2 levels. The drop checker (Red Sea) is light green and seems to be working correctly. I've seen it go to yellow when I made too strong a batch for my DIY fermentation canister and my fish gasping for air before . I've also seen it blue when the fermentation stopped. So I tend to believe I have the right level of CO2
I use two different KH and PH test kits which read consistently against each other, and read 10 degrees KH and PH of 6.6. If I read charts correctly, that would be 75 ppm.
My fish and cherry shrimp are thriving, the cherries are breeding and some young have grown to adults.
I use deionized water (API Tap Water Filter) remineralized with ElectroRight for water changes. KH reads 3 when I put it in the tank. But the tank always reads 10.
I don't use Peat, and I don't use PH adjusters. Phosphates read less than .25 ppm. I also use eco-complete as my substrate.
I can't make sense of this,
1) do the KH and PH readings make sense? Wouldn't a CO2 concentration of 75 ppm kill my fish and cherry shrimp?
2) What is likely driving the KH up?
3) Should I try to keep the KH lower? How?