Ista drop checker not working?

[Redzzz]

Hi all bros,
I set up my planted tank with co2 yesterday and have sticked a ista drop checker immediately after I turned on the co2. But until today it still shows blue colour in the liquid. I thought my 6ft tank did not have enough co2 and increased it, resulting in one of my tinfoil barb in my sump tank to go heaven.
I do not think I misread the instruction for installation. Any kind bros can advise?
thanks in advance.

[Shadow]

does the instruction asking you to dilute with tank water? If yes, use kh=4 solution instead of your tank water.