Seems like you're on the right track now. Hope it works out and you start to see an improvement soon.
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Is this the container that has salt residue in it? If so, it probably will never read 0 TDS due to dissolving the salt.
Btw, not all well water is high in CO2. I drill them for a living and the water can vary greatly. I used mine for years with no RO on it with great results. I have since put one on it but still do water changes with well water if I'm in a hurry.
Happy you found the culprit! Use some sort of macro algae or ATS and it will help pull the CO2 that's left out of your tank.
The Poly Filter Pad will tell you if you have any metals or anything in the water.
Did you mean 20 mV without a ground probe?