I tested the tank water diluted as per Red Sea instructions and got the same zero reading, as anticipated.Hi again, sorry I still need to clear up the question of zeroing the Hanna. Did you use the reference solution to zero the hanna before testing the same reference solution with the added reagent.
Or did you zero both times with the DT water?
With regards dilution test. I see your point with regards one would expect a much darker colour. The only reason I added this is that you mentioned the colour gradually faded and also mentioned a reading of 0. I am not a chemist so do not know the full process of the red sea test. However I have seen other tests show a false negative if the reading is off the scale. I thought I would just add the suggestion of diluted test just to eliminate that from the process.
Cheers, Tony.