Hello crad,
Well first the only tests I run on my
nutrient solution is pH. I use pH drops to do that because the drops wont give false readings. With the drops there is no need to calibrate, and/or do regular maintenance that all will affect readings. Also you simply don't need to measure the pH down to 1 tenth of a point, and the drops are plenty accurate enough, as well as easy to read results.
I also don't use a EC/TDS/PPM meter to test nutrient strength because I use the manufactures mixing directions as a general guide, and observe how my plants are doing daily. The only thing I really want a meter to do, their just not capable of doing. That is tell me what the concentrations of each individual element is, rather than just a total of everything in the water. Knowing if there's a element deficiency or toxicity is useful, but knowing the total amount of every element in the water wont do that. I also do regular nutrient changes to make sure the balance of the individual elements don't get to far out of balance. Meters simply cant tell you that important part. But If I had extra money to spend I wouldn't mind having a EC meter to use as well. Though at this point I would much rather spend a $100 on
nutrients.
Water quality
Well that's one of the most widely debated issues among growers. Personally I use nothing but RO water right now. But because of the low volume of water the RO system puts out, it wont be acceptable on a larger scale. They make large volume RO systems that produce 700 gallons of water per day for as little as $400. But another drawback to RO systems is that they waist water as well, even more than they produce. One of the benefits to growing hydroponically is that it takes a fraction of the water than growing in soil, but if your RO system waists 2 gallons of water for every gallon of useful water it produces, that triples and significantly increases water usage. For a large scale setup that I'm planning, I'm planning on using a series of inline cartridge filters that wont waist water. Including sediment, two carbon cartridges, and probably even an ion cartridges, as well as a cartridge with an absolute micron size of 1 micron to filter spores. And for the heck of it I may even add a UV light to kill any pathogens that may somehow make it past all the filters (which is highly unlikely with the absolute 1 micron filter).
Anyhow Here are a couple of articles about water quality:
Hydroponic H2O: Water Quality and Treatment
Water Wisdom For Hydroponics