Hi @LuvlyjubI know many dismiss the accuracy of test kits but I find JBL more reliable provided you follow the instructions and apply some common sense. Anyway my result was immediately dark blue and the scale of JBL only goes to 1.8ppm and my assumption was that this was high and possibly an issue?
I use the JBL PO4 (sensitive) test kit and it serves me well. As you have noted, the maximum reading that it can handle is 1.8 ppm. In order to measure in excess of this figure, it is necessary to dilute the tank water sample or tap water sample using RO/DI, low conductivity rainwater or distilled water. So, if you were to dilute the tank or tap water sample with an equal amount of RO/distilled water, that would be a factor of two dilution. From this diluted sample, you would still use 10ml in order to carry out the test. This principle can be extended to increase the upper PO4 limit. Now, let's say that this time around, the diluted sample matches the 1.2 ppm colour patch. You'd simply multiply this by 2, which would give you 1.2 x 2 = 2.4 ppm.
I hope that's clear.
Please let us know the outcome and then we can discuss this further.