You can try testing the water from the tap on the side of the well. See if it's different.
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When you test the water, do you rinse the phials out in some water first?
And do you use the stopper cap on the top of the phial or do you use your finger? Skin oils can affect the readings so use the stopped cap.
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The R/O (reverse osmosis) unit in the link looks alright. All R/O units create a lot of waste water, which isn't a problem if you have your own water supply. But for those paying for mains water, they waste 50% or more of the water that goes through them. Basically for every 1 litre of pure water they make, they create at least 1 litre of waste water.
They simply push water through various cartridges containing very fine membranes and other substances in an attempt to remove chemicals and minerals from the water. Good units will remove 99.9% of anything in the water leaving almost pure water with no mineral content and a neutral pH (7.0).
The minerals are removed with the membranes, whereas chemicals are removed with carbon. You can have ammogon/ zeolite and even nitrate absorbing substances fitted in them too, but normally it's just a prefilter to screen out course particles, then membranes and carbon.
You can buy all types of normal water filters to run inline with the R/O units and these can help the membranes and chemical removal media last longer. You just connect the normal water filters before the R/O unit and let the normal filter screen out the particulates and heavy metals leaving cleaner water to go through the R/O unit that can remove the minerals.
You should be able to change the media in them and use aftermarket stuff, especially for carbon and ammogon/ zeolite. Find a company online that sells Highly Activated Carbon or Activated Carbon (preferably highly activated) in 10kg (22pound) bags. Then use that to replace the stuff in the R/O unit. Get a small fine carbon too rather than big course chunks. Same deal with ammogon/ zeolite.
*NB* Rinse all carbon and zeolite products before using them to get rid of the dust on them.
Most units need to be back flushed regularly and you simply take the inlet hose and put it on the outlet and let it run for a bit. I don't know if they still do this but 20 years ago it was the norm.
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I wouldn't bother with a TDS meter. You have a pH and mineral issue and a TDS meter won't help with that.
You could get your water checked for salt (sodium chloride). Most good pet shops that sell marine fish have a refractometer to measure the salinity in sea water. They are pretty accurate but if there was any major salt in your water you should taste it.