Got some input for other people on the sodium mystery here was the best response. I also used the TDS meter on my tank and came back with a value of 550ppm.
That makes a lot more sense. I was thinking this was some sort of pool testing service you were using that gave you a comprehensive water quality report. They usually separate sodium from chloride in terms of ppm.
Reading the page for the current meter (
PoolMeter™), it looks like it really measures conductivity. It is calibrated against either a natural water standard (with a mix of ions) or a sodium chloride standard (salt only, or almost exclusively.) However, there is no way that a device of this type could distinguish among different ions in solution. That is, it is picking up everything in the water and calling it sodium chloride.
It appears that the NaCl value is a rescaling of the TDS value, or vice versa, depending on the calibration. The reason that the two scales are different is that it takes less NaCl to reach a given conductivity as compared to a generic mix of sodium, chloride, sulfate, and carbonate ions (the TDS mixture it assumes you're testing.)
A straightforward way to test this idea would be to dissolve 0.25 grams of salt in 1L of RO water and 0.25 grams of KNO3 in another liter of RO water. If I'm right it should read ~250 ppm NaCl in the first and somewhat less (~175 ppm NaCl, at a guess) in the second. Similary, the TDS measurements would be ~450ppm and ~225 ppm, respectively. If it truly is a TDS meter and an NaCl meter, it will read 250ppm TDS in each, 250ppm NaCl in the first, and 0 ppm NaCl in the second, but I can't see how the device could make that distinction from what I can see of its construction.
All of that said, that does still mean that you've got a fair bit of stuff dissolved in your tank. What that is remains a mystery at this point.
