As best we can tell, metals testing used on pool water is NOT reliable.

One problem is that is that various pool products contain chemicals that can 'shield' soluble iron or copper from being measure. A second problem is that the tests only detect solubles, and simply the process of collecting a water sample, and carrying it to the store can change soluble metals to insolubles. When I used to collect water samples on a potable system, the sample used for metal testing had to be 'pre-charged' with acid and also had to be filled to eliminate ALL air space in the sample bottle.

The metals bucket test is intended to 'break' any 'shielding', drop ALL the metals present to the bottom, and give a reliable qualitative indications of metals present, rather than the quantitative, but unreliable results from various test kits.