How accurate are the phosphate test kits? The ones I have has the tube that you fill with water then drop the tablet in & compare the blueness once the tablet disolves.

Some may remember that I have continual phosphate problems which leads to algae regardless of my chlorine levels. I check the phosphate level of my tap water yesterday & came up with the darkest blue on the comparator which signifies 2500. Just wondering if that were accurate or not!