interpreting salifert test kit reading

[Potsy]

I do realize that this isn't the most accurate way of measuring phosphate. But, when I compare the sample color to the chart, I find that my sample is a milky, grey color without any blue. I'm wondering should the color of the lowest reading be perfectly clear water or greyish/cloudy but not blue?

[disc1]

That means Phosphate is lower than you can test with that test. ie. probably not time to change the GFO yet. If you want better results at lower concentrations, look into the low-range hanna instrument.