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Good points, especially the one about checking multiple times per year. Depending on the source(s) for your municipal supply - there could be very large differences seasonally or even week to week. One example that I've seen is a town getting its water from a mountain stream for winter & spring but deep wells for summer & fall. The TDS was a step function between the two. A TDS meter & periodic testing is a cheap insurance policy (it also will let you check the rinse water of your sand or perlite to see how many ppm comes from them when mixing up new media ....
)As others have noted, this tends to be in a range that most consider acceptable. However, I usually like to think a bit about 'rules' that others use and how they fit my particular situation. For example, as another poster noted - lets say 100ppm marks the range for 'unacceptable'. If that's true and your water remains ~20-25ppm, you'll have 4-5 times as long before your media becomes as 'unacceptable' as if you were using the 100ppm water. It's not that your water is permanently ok. Of course, you must also consider your conditions and the plants you grow. Some neps & pings seem to like hardness (to some degree). Are your plants outside where they get a periodic rain flushing or are they permanently inside where water evaporates everyday - concentrating the salts?
