GH and KH Explained: The Hardness Numbers That Matter

The short version

• GH (general hardness) is the dissolved calcium and magnesium in your water, the minerals fish drink and shrimp build shells from. KH (carbonate hardness) is the buffer that stops pH from swinging.
• Both are measured in degrees (dGH and dKH) or ppm; one degree is about 17.9 ppm. Your tap already has its own GH and KH before you add anything.
• GH decides which fish and shrimp suit your water: soft-water neon tetras want low GH, hard-water guppies want high. KH decides how stable your pH is.
• Test your tap first with a GH/KH kit, then match your stock to it. Choosing fish that fit is far easier than changing your water.

Two tanks sitting at the exact same pH of 7.0 can be right for completely different animals, because pH hides the two numbers that actually matter. Those numbers are GH and KH, the hardness readings, and they decide whether a shrimp can molt and whether your pH will hold or crash.

A test strip that shows only pH is telling you the least useful third of the story. Here is what each hardness number means, how they connect to pH, and how to read your own water before you buy a fish.

GH: the minerals fish and shrimp are built on

General hardness is the amount of dissolved calcium and magnesium in the water, and it is not optional for the animals. Shrimp pull calcium straight out of the water to harden a new shell after each molt, and too-soft water gives them failed molts. Fish osmoregulate against the mineral level they evolved in, which is why soft-water and hard-water species do poorly in the wrong end.

The ranges are specific. A cherry shrimp (Neocaridina davidi) wants GH 4 to 14, a neon tetra (Paracheirodon innesi) 1 to 8, a guppy (Poecilia reticulata) 8 to 20, and a nerite snail (Neritina sp.) 6 to 18 or its shell pits and erodes. Match the fish to your GH and most of stocking takes care of itself.

KH: the buffer that keeps pH from crashing

Carbonate hardness has nothing to do with minerals fish drink and everything to do with stability. KH is the carbonate and bicarbonate in the water, and it works as a buffer: it neutralizes the acids a tank constantly produces, holding pH steady.

When KH sits near 0 to 1 dKH, there is nothing to absorb those acids, and pH can crash from 7.0 into the 5s overnight, which is far more dangerous than a stable low pH. At 4 dKH and up, the buffer is strong enough that pH barely moves week to week. KH is quietly the more important of the two numbers, because a pH crash from zero buffer kills faster than water that is simply hard or soft.

How GH, KH, and pH connect

The three move together more often than not, which is why keepers confuse them. Water that is soft (low GH) and poorly buffered (low KH) tends to sit at a low, acidic pH, the blackwater end where neon tetras and Caridina shrimp are most at home. Water that is hard (high GH) and well buffered (high KH) tends to sit alkaline, above pH 7.5, the range livebearers and snails want.

KH is the lever on pH: raise KH and pH rises and stabilizes with it, which is why adding a carbonate source lifts both. You can have exceptions, soft water that is buffered or hard water at neutral pH, but the tap you start with usually sets all three at once.

Reading your tap and your tank

Test your tap water first, before you buy a single fish, with a GH/KH test kit. Tap varies enormously by city: some homes run 2 dGH out of the faucet, others 20, and it decides your entire stocking list for free. Here is the rough map from a reading to what suits it.

KH is not a species requirement the way GH is; it is the buffer that usually comes alongside a given hardness, so soft-water fish generally sit in low-KH water and hard-water fish in high-KH water. Read your tap, then pick from the row it lands on.

Changing hardness the stable way, if you must

Choosing fish that fit your tap is easier than changing your water, but sometimes you want a specific animal. To raise GH and KH, a bag of crushed coral or a seiryu stone in the tank slowly dissolves calcium carbonate, lifting GH, KH, and pH together over weeks. To rebuild hardness in RO or very soft water, a shrimp remineralizer adds back exact minerals: a GH/KH+ blend for Neocaridina, a GH-only blend for Caridina that keeps KH near 0.

To lower hardness, dilute tap with RO water, or use an active substrate like aquasoil that binds minerals and pulls pH toward 6.5. If you want a rock that changes nothing, dragon stone is inert where seiryu is not. Do it slowly: a sudden swing is worse than the wrong stable number. Our guides to lowering pH and raising pH and hardness cover each direction.

The honest part: chasing numbers

The classic disaster is a pH crash from zero KH. A keeper reads a low pH, tries to raise it, ignores KH, and the unbuffered tank swings wildly and kills fish faster than the original low pH ever would. The second mistake is running RO water for soft-water fish but never remineralizing it, leaving shrimp with GH so low (under 3 or 4 dGH) that they die mid-molt.

The third is dropping a seiryu stone into a soft-water shrimp tank for looks, then wondering why GH and pH keep climbing out of the shrimp's 4 to 14 range. Stable and roughly right beats perfect and swinging, every time. Pick water your animals fit, and hold it steady.

Frequently asked questions

What GH and KH should my tank be?

It depends entirely on what you keep. Soft-water fish like neon tetras want GH 1 to 8 and low KH; hard-water livebearers like guppies want GH 8 to 20 and higher KH. For a general community, GH 8 to 12 and KH 4 to 8 is a stable, forgiving middle. There is no single correct number, only the number your animals need.

What is the difference between GH and KH?

GH measures calcium and magnesium, the minerals fish and shrimp use biologically. KH measures carbonates, which buffer the water and hold pH steady. GH decides what can live in your water; KH decides how stable that water is. They often rise and fall together, but they are separate measurements and a test kit reads each on its own.

Does KH raise pH?

KH stabilizes pH and tends to hold it higher. A high KH keeps pH up in the alkaline range and stops it dropping; a near-zero KH lets pH crash. Raising KH with crushed coral or a carbonate buffer will usually raise pH and, more importantly, keep it from swinging.

Can I keep shrimp if my water is very hard or very soft?

Match the shrimp to the water. Neocaridina like cherry shrimp handle a wide GH 4 to 14 and even hard tap, so most homes suit them. Caridina like crystal shrimp need soft water (GH 3 to 6) over a buffering substrate, so hard-water keepers must use RO and remineralize. Forcing the wrong shrimp into your tap is the common way to lose a colony.

The move that saves the most heartbreak is the cheapest one: test your tap, then choose animals that already fit it instead of fighting your water every week. From here, what pH means in an aquarium explains the number KH controls, and water hardness for aquarium plants covers what your plants want from GH. Plan a stocking list around your real tap numbers with the build planner, check the GH range of every species in the compatibility database, and browse the rest of the water chemistry guides at the hub.