What is nice is I can walk away for 15min and come back and the tubes will not have gotten any darker or lighter so it seems as if as long as you read them after 5min you are okay up to who knows maybe an hour?
I only clean the test tubes after the rack is full.
Sometimes for comparison after WC's.
NH3, NO2, & NO3 are unchanged for an unlimited time.
PO4 turns dark and cloudy with particulate matter/coagulation.
Ca test turns clear after time.
GH & KH will continually change due to minute evaporation.
If we are testing multiple tanks, as some of us have many.
You can't go wrong.
NH3 tests are extended from 100 to 400
NO2 tests are extended from 180 to 900
NO3 tests are extended from 90 to 450
PO4 tests are extended from 150 to 450
1% of the time I get a false positive that requires a full sample test.
The hardest part is getting used to rinsing pipette, cylinder, and test tube with sample water before applying reagents.
This turns 4 test kits into an equivalent of 17.
Depending on place of purchase it's still a good bit of money.
Back to the core question.
Have you finally decided to purchase KNO3 and dose to meet your needs?