The short answer
TDS (total dissolved solids) is a measure of everything dissolved in your water — minerals, salts, trace elements and organic waste — read in ppm by a cheap handheld TDS meter. It gives you a single quick number for how “loaded” your water is. It’s popular with shrimp keepers because it’s fast and sensitive, but it tells you the amount dissolved, not what it is — so it complements GH and KH testing rather than replacing it.
How a TDS meter works
A TDS meter measures your water’s electrical conductivity and converts it to an estimated ppm figure. Pure RO water reads close to 0; tap water might read 150–400+; a mature planted tank sits higher because of accumulated minerals and waste. It’s a two-second check that instantly flags a change — a rising TDS between water changes hints that waste is building up.
Why shrimp keepers love it
For shrimp, stable water is everything, and TDS is the quickest way to spot drift. Keepers mix RO water up to a precise TDS with a remineraliser, then watch that the number stays put. A sudden jump or drop can trigger bad moults. It’s less critical for hardy community fish, but still a handy early-warning tool.
TDS vs GH and KH
TDS overlaps with hardness but isn’t the same: GH and KH measure specific minerals, while TDS lumps all dissolved solids together, including things hardness tests ignore. Two tanks with identical GH can show different TDS. So test hardness with a liquid test kit for the detail, and use TDS for a fast daily pulse-check.
See KH and GH explained, using RO water, and the water testing hub.