FYI,
In reefing there's a new trend of send your water out for detailed "ICP tests". One vendor, ATI, will also test your RODI water for free with a test (reefers definitely should be using RODI or distilled water)
So after getting lots of "your RODI is fine nothing measurable" results, I decided to send Arlington tap water, after it gets to my building. The results were interesting. (those who live in DC and Falls Church IIRC get water from the same plant)
Most parameters were zero (even, oddly, nitrates), except for the following:
Silicon 3100 µg/l (supposedly from a quick google search "The silica content in natural waters is commonly in the 5 to 25 mg/L range, although concentrations over 100mg/L occur in some areas")
Barium 23.49 µg/l
Copper 3.37 µg/l. (far below ostensibly dangerous levels, it seems to me from a google search. Arlington county's 2018 water quality report says that their range of detected levels were 0.01 – 0.19......hmm....... piping problem somewhere?)
Zinc 6.45 µg/l
Phosphorus 710.3 µg/l Phosphate 2.17 mg/l
Aluminium 21.97 µg/l. (note: according to the WI department of Health, "the World Health Organization has recommended that aluminum levels in community water supplies should not exceed 0.2 mg/L.". So this theoretically should be fine for drinking water, it seems)
This is obviously not what we want for reefing. I don't even know if I'd want any of this in a simple freshwater tank, though only phosphate and Al look like they're problematically high levels......