Instead of a distinct blue endpoint the color changes to a purplish color and might have purple 'floaties' in the blue solution. By adding a small amount of the third reagent first you tie up the metals and prevent this. I have used up to 6 drops of the third reagent in this manner when testing with excellent results. The color change is much more distinct. Remember to count the drops added before the indicator solution as part of your total!

I don't use metals in my pool - but the purple floaty things is what I get when I test for Calcium - should I have my water tested for metals . I use Jack's purple stuff already as required by my pool builder for warranty - but this worries me - all fill water is city tap water.