I am measuring my CO2 with my aquarium water, my KH and PH readings.
I don't know what a drop check is. Could you point me where to find out about it and which one you use, etc?
My PH has been 6.4 (35ppm C02 per table) brought it down today to 6.6 since fish were ok at that PH level ( so now CO2 should be at 23ppm)
My KH ..53.7ppm (3 degrees K)
My Nitrate today 20ppm (I just dosed fertilizers including KNO3)
My fish were happy Oct. 27 for sure, changed 50% water and added, for the first time: 1/4 tsp MGSO4(EPSON SALT) and
30ml of Excel. Obviously I overdosed. Have stopped using excel until I understand it.
At this point some fish died off.

Today I have installed a 25 Watt UV sterilizer to help with diatoms and green algae. BBA has started to appear now.

Your opinions would be appreciated