To hammer that point home:
I only know what's in those specific tanks because I carefully measure what goes into the tanks. Okay, "carefully"
instead of just carefully - it's as close as I can get with a decent gram scale. Obviously can't measure waste beyond seeing it and generally monitoring, say, nitrates. And even then who knows precisely without some deeper science that I am too exhausted to consider?
The amounts of CaSO4, MgSO4, K2SO4, FeSO4, MnSO4 and a couple other things used are measured for my DIY shrimp mineral mix. It's essentially the same thing as the more popular remineralizer mixes but with a couple extra things I add (traces and some humic goodies) in addition to the commercialized mix. I use the commercialized mix on some tanks because I have a lot of it. But the rest? I use my own stuff because it's exceedingly cheaper - $10-$12ish for roughly 4 pounds vs $175-$200 for the same amount of the commercial stuff. The price we pay for convenience when we care about something, right?
Pro-tip: The only reason people like me recommend the commercial stuff is because it's easy and tough to mess up. Paying a premium is sometimes worth it if one isn't keen on constantly measuring (guilty!) out your mix each time you use it. (Easier to measure DIY as-needed because it's easier to store mineral salts in the bags they come in.)
Sometimes I use calcium carbonate or baking soda to the mix and that goes into my community tanks. That's generally at a higher concentration than for simple shrimp tanks. TDS in those tanks is probably 230-240? Maybe more? I know it's higher than the water I mix up but haven't actually measured in long enough that I'd have to check notes.
Unfortunately, I can't be sure what's in my tap water so it's only in used in larger tanks or container ponds. It's one of the reasons @Zoidburg
tells people to be careful if not wary - because tap water can be unpredictable when it's not clear what goes into that water. You can kind of
get an idea by looking at municipality water reports if you're on a public utility system. But even that gets wonky if your city's pipes are, say, 150-years-old (which is why my city experiences major water main breaks every five seconds but I digress) and who even knows what comes outta the pipes then? The best I can do is guess in that case and I rarely if ever use that water for anything super-sensitive like shrimp.
When I lived in a newer part of town and closer to a different pumping and treatment facility, tap water was much more consistent. Same for my office. But my new place? Not so much.
Oh, almost forgot. In tanks where I heavily fertilize - or used to - they would have TDS in the 400 range. That's with calibrated meters and actual TDS. But I had a rough sense of what made up those dissolved solids because I added most of them to the tanks in the form of dry or pre-mixed fertilizer.
I guess the TL;DR here is this:
TDS is probably only useful if you have some idea of what makes up your water, if you're keeping an eye on your RO/DI system or if you're using it as a means to loosely monitor for an "OH NO!" moment in your tank.
Numbers, ratios, measures- they have context.