Thanks for reply.
I was wrecking my mind with this, but doubting my understanding on the subject which is as you concur.

I ordered 4 bottles of regent and each with the same result: amber/yellow and does not change color in the tank. The tank has, at least, 19ppm CO2.

Is it allowed in this forum to warn other by giving out the name?

raul

PS Here is a picture of the DC 6 hrs in the tank.