GH, KH, and TDS: three dials people keep conflating
GH, KH, TDS get used interchangeably in half the forum threads I read. They measure different things, respond to different fixes, and confusing them is how people end up pouring pH-up into a tank that was fine. Here is the shortest accurate version I can write.
GH: the minerals that build things
General hardness is dissolved calcium and magnesium. Calcium goes into shrimp exoskeletons and new plant cell walls; magnesium sits at the center of chlorophyll. Low GH means shrimp molt problems and pale new crypt leaves. The fix is mineralizing — a remineralizer product or, in my case, halves of cuttlebone-free messaging: a small bag of aragonite gravel in the filter pushed my soft tap water from GH 3 to GH 7 in a week and held it there. Stable beats ideal.
KH: the buffer, not a nutrient
Carbonate hardness measures carbonates and bicarbonates — the buffer that resists pH swings. Plants do not eat KH in any meaningful way. High KH means your pH refuses to move; low KH means one overfeed can swing the tank. The classic mistake is "fixing" a low KH with pH-adjusting chemicals, which fights the buffer all day and loses. If your pH is stable and your stock is fine, your KH is fine. Full stop.
TDS: the total, and the shrimp sellers' crutch
Total dissolved solids is a conductivity reading — literally everything dissolved: the good minerals, the plant food, the Remineralizer, the leftovers of the last medication, and the tap water's own travel history. It is the most useful number in the hobby and the least informative at the same time: TDS tells you something changed, never what. My rule: watch TDS drift between changes (more than ~20 points means something is accumulating or something evaporated unevenly), but diagnose with GH/KH tests, not the pen.
How they interact
| Symptom | Check first | Usual fix |
|---|---|---|
| Shrimp molt failures | GH + TDS swing | Remineralize consistently, match change water |
| pH swinging day to day | KH | Leave it alone unless stock objects; stop pH products |
| Pale new growth all over | GH + fertilizers | Root tabs and all-in-one, not KH adjustment |
| TDS creeping up weekly | Evaporation vs accumulation | Top up with RO, not tap |
The temperature connection
One crossover with the crypt melt post worth repeating: a large temperature-mismatched change can melt crypts without moving GH/KH/TDS by a point. Osmotic and thermal shock are different animals; matching change-water temperature fixes the thermal one and costs nothing.
Test GH monthly, KH when something feels wrong, TDS weekly as a smoke detector. Diagnose with the test kits, act on the change water.