This is all well and good, but testing TDS is a waste. Here’s why.
TDS will always be higher than your hardness. Period. How much higher depends on environmental conditions, such as amount of rain, what kind of stone the water passed through on the way down, how many organics have leached into the system, any man-made chemicals that have leached into the water, the depth of the well, etc. This is because a TDS meter measures salts, organic material, pollutants from fertilizers, pesticides and herbicides, and ions such as iron, cesium, selenium, arsenic, copper, etc. above and beyond what a hardness test measures.
Which brings me to my first point on why TDS testing is a waste of time and money. Since so many factors can affect the TDS, TDS does not consistently scale with hardness even at the best of times. With a well, it’s even worse from one day to the next. You may have 50 ppm above your hardness measurement one day, 200 ppm the next, and 25 the next. This being the case, you pretty much have to run the test at least daily to maintain even a slight modicum of relevance.
Secondly, although this all sounds well and good in theory, there another flaw: it’s not possible to distinguish between the different compounds in the water by any means practical to hobbyists. The TDS measured is a general conductivity measurement that depends on the overall amount of chemicals (using chemicals as an all-inclusive generalized characterization of the materials listed above, and more) dissolved in the water. However, a simple TDS test doesn’t tell you how what those solids are. You can’t determine if the additional dissolved solids are iron or other mineral, dissolved salts, dissolved organics, pesticides or fertilizers that have degraded, etc. Therefore, you cannot know how much of the content included in the TDS is made up of each component. You don’t know whether to correct for excess iron or a pesticide, for high organics or excess salts, etc. You’ve simply verified how much higher the total dissolved solids are than the hardness. You don’t know WHAT those additional chemicals are. Yes, the percentage content can be determined, but not with any tests practical for a hobbyist. And again, due to changing environmental factors, you would need to test often to maintain relevance.
So when it's all said and done, all you're really doing is confirming that your TDS is higher than your hardness and by how much. It's a given that your TDS is higher, and how much higher is irrelevant if you don't know the composition and you can't make adjustments to correct any imbalances.
I agree on the water softener. I’ve known folks who have kept plants and fish long-term in softened water. However, most were casual hobbyists with a tank or two, keeping common hardy fish and plants, who weren’t trying to breed anything, and weren’t trying to keep delicate plants or fish. Freshwater fish try to retain salts and eject water from their bodies but I suspect long term the excess salts could be harmful, especially to the kidneys and gills, due to the additional excess salts that would need to be processed. Anyways, I’m on a well, and have the proverbial liquid rock for water. My wife wanted a water softener when we moved in 16 years ago, but realized the possible effect on my fish and we did not get one. My wife is so good to me.
I decided the best bet was to buy livestock that would thrive in my water. Of course, most of the fish I like fall into this range anyways.
I went through my “test-anything-and-everything” phase 30 years ago. After a while I realized a lot of the things people test for really don’t do a heckuva lot more than waste my money and time. Most long-term hobbyists have gone through the same phase and many have come to the same realization. If you have rare and/or delicate species then maybe you do need to test more parameters. And some people just like to tinker. But I’m a firm believer in the KISS method. I keep track of the nitrification parameters, pH, alkalinity, hardness and temperature regularly, and generally don’t worry about too much more.
WYite