We test hardness because it helps metal toxicity by out competing on the gill uptake sites on fish. SO the harder the water, the more metals that can be in the water. Trout need a range of 6 to 8 to not be stressed. We test this using a Beckman pH meter with an automatic temperature compensator. The CDOW runs the test at their lab in Fort Collins.
Metals are around in most rivers in trace amounts.
Copper is essential for cellular metabolism and the transport of oxygen, as well by some enzymes. It will become toxic at 20 ppb. Iron is essential in small amounts, toxic at 1,000 ppb. Lead is not essential, toxic levels are thought to be in the 10 - 100 ppb range. Zinc is essential for cell growth and differentiation.