TDS can be useful in terms of gauging the total amount of "stuff" in one's water. But to be most useful requires we know a lot about both our tap water and then out tank water parameters.
I have cyc;ed filters rater than tanks for some time. But that was because I am usually not setting up single tanks. Either I was setting up 6-8 summer tanks and/or taking 4-6 tanks when I was selling from either the vendor room or my hotel room at a weekend event. So, I ran a bio-farm for this purpose. I ran the farm using an API ammonia test kit and a TDS meter.
I needed to be in control of only two factors= ammonia and KH. The bacteria use carbonates and bicarbonates as a source of inorganic carbon and get used up easily when cycling filters for 8 fully stocked tanks holding between 11- and 50 gallons. SO I need filtration for about 200 pr more gallons. This was done using a single 40B for all the filters.
The dosing of ammonia controlled the reproductive process for the bacteria and Archaea and I needed to keep the ammonia at no more than 6.5 on the API kit or risk stalling the cycle or developing the wrong strains of bacteria. The start of the process is easier. My goal was to have the filters cycled to process at least 3 ppm of ammonia in 200+ gallons of water. Byt the end of the cycling I had o be adding in between 20 and 25 ppm of ammonia to the farm every day.
I had to do this amount in several smaller additions several times/day. So I was doing a lot of testing for ammonia and KH. I did the latter via my TDS meter. I could do this because I knew that I could expect to see TDS changes driven more by changes in the KH than anything else.
What controls pH in a tank is KH. And a drop in the pH into acid territory slows a cycle. So, if I saw rhe cycle slowing I knew to test TDS and there I would determine if I needed more crushed coral in the tank to raise the KH. TDS are a lot faster and cheaper to test than KH and pH.
So, in this application I was basically able to use the TDS meter to control both the KH and pH without needing to test them.
My goal was to have all the filters cycled and ready to go in fewer than 2 weeks. But to hit that target or even just a dozen days, everyhting had to proceed without errors. But I have well water and the parmaeters are fairly stable under normal rain conditions. Periods of drought or heavy rains for a number of days would alter the TDS. But this is a somewhat slow process and so was easy to know what to expect.
This is not the usual circumstance for using TDS as a reliable measure of what is going on in a tank, especially if if is cycled and stocked and especially planted. However, I found TDS was a big help for breeding plecos. I used it to control a simulated dry/rainy season of several months in a tank. The absolute TDS were not as important as the change in them between the pek of the dry season and the onset of the rainy season. Basically, one season should be my tap and the other reqquired altering the ywo extremes. The rule I used was that the peak of the dry should be about double the low of the rainy.
To avoid using RO or RO?DI int this process I chose to use my 83 ppm tap as the rainy and them I used magnesium, calcium and a pnch of naking soda to harden my tap into the 170 ppm range for the peak of the dry. This was pretty easy to manage using TDS because i knew what I waa adding to the tap water tp harden it up.
I would not be inclined to rely on TDS for specific parameter information unless it was a well controlled situaltion like a bio-farm. The more parameters one needs to stay on top of, the less usefull TDS becomes.
I have measured my TDS from the tap as low as 53 ppm during periods of frequent heavy rains and as high as about 120 ppm during periods of drought. But what was causing these extremes I could not determine from the TDS alone.
Finally, nitrate testing is the least accurate one we do. This test is least accurate between 0 - 20 ppm. Plus, most kits actually measure nitrate by first converting it to nitrite and then measuring that. Of course, if there is nitrite in the water, this will get counted as nitrate.
One the other hand when I rank a high tech planted tank with pressurized CO2 I used a lot of testing and almost no TDS to manage the tank parameters. The only problem with nitrate there was having to add it. O nitrate is not great for plants. Since we cannot add amonia to feed them, we have to use nitrate as a nitrogen source as at lower levels it is fish safe.
I hope this above is helpful in understanding how we can use TDS ot not for managing specific things in out tanks. The more detail we need tyo know, the less usefull TDS becomes. Ad there is not economical way we can measure nitrate save our test kits. The best use for them it they can give us good directional information- are they rising or dropping. But for abosulte levels we need to accept that the kits can be wrong ro some axtent.
Bear in mind that science telss us to keep nitrate under 5 ppm for cycling but that is using the nitrogen scale which only counts the Ns in NO3. Hobby kits measure using the Total Ion scale which counts the 3 Os. So the number on an API kit to be under 5 ppm N03-N would be 22 ppm on an API kit. The USA often recommends that the number for nitrate-N in drinking water from out tap should be under 10 pp, which is 44.25 ppm on an API kit.
Referance for the conversion between the two different scales can be found here
https://www.hamzasreef.com/Contents/Calculators/NitrogenIonConversion.php
