A tds test is not reliable as a test for gh or kh. tds tests for all disolved solids by checking the electrical potential of the ewater, as such, it even registers salt content. Gh tests for mainly calcium and magnesium. Kh is carbonate hardness. All three need to be done, and not a single one can be trusted as a test for all three, as they test different things. TDS is a good start as it shows overall conditions, but it does not tell you what exactly is in your water. So you can have mineral depleted water, with not a trace of calcium, magnesium or anything else in it, but it has a lot of salt in, and have other water with nothing except a very high level of calcium, and both can give off the exact same TDS reading... So lesson is, don't trust one for all. I myself would really love a gh tester, and a kh tester, as so far I have only found electronic testers for pH and TDS.