GH and KH — aquarium hardness, finally clear.

Water hardness confuses more aquarists than any other parameter, for an understandable reason: there are two kinds, they sound similar, and they are quoted in a tangle of different units — dGH, dKH, ppm, mmol/L. But GH and KH measure genuinely different things, each matters for different reasons, and once the distinction clicks, a lot of murky advice about pH and shrimp and "soft versus hard water" suddenly makes sense. This guide separates the two cleanly. The GH/KH converter handles the unit math and includes a species hardness reference.

GH and KH Are Not the Same Thing

GH — general hardness — measures the total dissolved minerals in the water, mainly calcium and magnesium. This is "hardness" in the everyday sense: how mineral-rich the water is. GH matters for the biology of your animals, because fish and especially invertebrates use these minerals for bodily functions — shrimp need calcium to build their shells, and livebearers need adequate minerals to thrive.

KH — carbonate hardness — measures the carbonates and bicarbonates, which act as a pH buffer. KH has little to do with minerals fish "use" and everything to do with pH stability: it is the water's capacity to neutralize acids before they move the pH. High KH means a rock-steady pH; low KH means a pH that drifts and can crash.

They often rise and fall together because both come from dissolved rock, so "hard water" is usually high in both. But not always — and the cases where they diverge are exactly where problems happen. A tank can have plenty of GH but low KH, leaving fish well-mineralized but the pH dangerously unstable.

The one-line distinction: GH is the minerals your animals' bodies need. KH is the buffer that keeps your pH from crashing. Different jobs, different consequences when they go wrong.

Why KH Controls pH Stability

KH is the hidden hand behind almost every pH problem, which is why the pH buffer guide and this one are two halves of the same topic. Acids constantly enter a tank — from fish waste, from CO₂, from decaying matter, from nitrification. KH neutralizes them. As long as there is buffer, the pH holds. When the KH is used up, the next acid input drops the pH sharply, and a low-KH tank can crash overnight, killing fish.

This is why "my pH keeps falling" is almost always a KH problem, not a pH problem. The fix is to raise KH — with crushed coral, aragonite, or a little baking soda — so there is buffer to hold the pH up. And it is why fighting to lower the pH of a high-KH tank is so frustrating: the buffer keeps pulling pH back up, and you are battling chemistry instead of working with it.

Why GH Matters for Fish and Shrimp

GH is the parameter that decides whether your animals can build and maintain their bodies properly. Dwarf shrimp are the clearest case: neocaridina (cherry) shrimp need adequate GH to molt successfully, and a GH that is too low causes failed molts and death. Caridina shrimp want specific, often softer parameters. Livebearers — guppies, mollies, platies — are hard-water fish that thrive on high GH and struggle in soft water. Conversely, many soft-water species — wild bettas, many tetras and catfish — evolved in mineral-poor water and breed best when GH is low.

So GH is a major compatibility and success factor. Matching a species' GH preference is part of choosing fish that suit your water, the approach laid out in the water parameter guide. The fish compatibility checker factors hardness overlap into its scoring for exactly this reason.

The Units, Untangled

Hardness gets reported in several units, and converting between them is where the GH/KH converter earns its keep. The common ones: dGH/dKH (German degrees, the hobby standard), ppm or mg/L as CaCO₃ (parts per million, common on test strips and lab reports), and mmol/L (used in some scientific contexts). The key conversion to remember is that 1 degree (dGH or dKH) ≈ 17.9 ppm. So a tank at 8 dGH is about 143 ppm, and a strip reading 120 ppm is about 6.7 dGH. Mixing up the units leads to real errors — thinking your water is far softer or harder than it is — so always confirm which unit a source or test uses.

Adjusting Hardness

Raising hardness is straightforward: crushed coral, aragonite, or mineral additives (including specialized shrimp remineralizers) add GH and KH. Wonder shell and seashells do the same slowly. Lowering hardness is harder and usually means diluting with reverse-osmosis (RO) water, which strips minerals so you can remineralize back up to a precise target — the standard approach for soft-water and shrimp keepers who want full control. As with pH, move gradually; sudden hardness swings stress fish and can be fatal to shrimp mid-molt. And note the overlap with salinity: adding aquarium salt raises certain dissolved solids but is not the same as raising GH, a distinction the salt dosage calculator keeps clear.

How to Test Hardness Accurately

Knowing your hardness starts with measuring it, and the method matters. Liquid titration kits for GH and KH are the hobby standard and the most reliable: you add reagent drop by drop until the water changes color, and the drop count gives the reading in degrees. They are cheap, accurate, and worth owning. Test strips that bundle hardness with other parameters are convenient but notoriously imprecise for GH and KH specifically, often reading a full degree or more off — fine for a rough check, risky for decisions. TDS meters measure total dissolved solids, which correlates loosely with hardness but is not the same thing: TDS lumps in salts, fertilizers, and other dissolved matter, so a TDS reading is a useful trend indicator for shrimp keepers but not a substitute for a true GH/KH test.

Test your source water as well as your tank, because tap water hardness varies by region and season and sets the baseline you are working from. Knowing your tap GH and KH tells you immediately which fish suit your water with no intervention — the cheapest path to success, as the water parameter guide lays out.

A Worked Example

Suppose your test strip reports water hardness as "120 ppm" and a care sheet says your target fish wants "around 8 dGH." Are you in range? Convert: 120 ppm ÷ 17.9 ≈ 6.7 dGH — slightly below the 8 dGH target, but close enough that a hardy species adapts fine, while a sensitive shrimp might want a small remineralizer bump. Now suppose your KH tests at 2 dKH (about 36 ppm). That is low, which explains why your pH has been drifting downward between water changes: there is little buffer to hold it. Raising KH toward 4–5 dKH with a touch of crushed coral would stabilize the pH without you ever "treating" the pH directly. This is the everyday payoff of understanding hardness — the GH/KH converter does the unit math so you can read every source consistently and diagnose problems like this at a glance.

Frequently Asked Questions

What is the difference between GH and KH?

GH (general hardness) measures dissolved minerals like calcium and magnesium that fish and shrimp need biologically. KH (carbonate hardness) measures the carbonate buffer that keeps pH stable. GH is about your animals' bodies; KH is about pH stability.

Why does my aquarium pH keep dropping?

Almost always because KH is too low. KH is the buffer that neutralizes acids and holds pH steady; when it runs out, pH falls and can crash. Raise KH with crushed coral or a small baking-soda dose to stabilize the pH.

What hardness do cherry shrimp need?

Neocaridina (cherry) shrimp generally want a GH of around 6–8 dGH and a KH of about 2–4 dKH for successful molting and breeding. Too-low GH causes failed molts. Use a shrimp remineralizer with RO water for precise control.

How do I convert dGH to ppm?

Multiply degrees by about 17.9 — so 8 dGH ≈ 143 ppm of CaCO₃. The same factor applies to dKH. The GH/KH converter handles dGH, dKH, ppm, and mmol/L conversions and includes a species reference table.