GH and KH for shrimp, explained
GH and KH are the two water numbers shrimp keepers talk about most, and they are easy to mix up. Both are called "hardness", both are often measured in degrees, and many tap waters have similar values for each. But they measure different things and matter to shrimp for different reasons. GH is about the minerals shrimp use to build a new shell; KH is about how steady your pH stays. This guide explains both, how to read and convert them, and how to change them without the sudden swings that cause most losses.
GH: the molting minerals
GH (general hardness) measures dissolved calcium and magnesium. Shrimp grow by molting, and they harden each new shell with minerals from the water and their food. That is why GH is the first number to check when shrimp struggle to molt. Hobby references commonly link GH that is too low to soft shells and stuck molts, and GH far above the usual range is also cited as a molting problem. See shrimp dying after molting.
KH: the pH buffer
KH (carbonate hardness) measures carbonate and bicarbonate. These neutralize acids, so KH acts like a pH shock absorber. In water with a few degrees of KH, pH tends to sit near neutral or above and stays steady. In water with almost no KH, pH can drop as acids from waste, plants, wood and soil build up, or move around as CO2 levels change. KH also drifts down over time in a tank as it gets used up, which is one reason regular water changes matter.
That is why the two shrimp groups want different KH. Neocaridina such as cherry shrimp are usually kept with a little KH so pH stays around neutral. Caridina such as crystal red and bee shrimp are usually kept at near-zero KH on an active soil that holds pH slightly acidic; any KH in the water works against the soil and wears out its buffering faster. See best substrate for shrimp.
GH vs KH at a glance
| GH (general hardness) | KH (carbonate hardness) | |
|---|---|---|
| Measures | Calcium and magnesium | Carbonate and bicarbonate |
| Why shrimp care | Minerals for building new shells | Keeps pH steady |
| Too low | Commonly linked to soft shells and failed molts | pH can drop or swing |
| Too high | Also cited as a molting problem; unsuitable for Caridina | Holds pH high; exhausts active soil quickly |
| Raised by | GH remineralizers, crushed coral and other calcium sources | GH/KH remineralizers, crushed coral, bicarbonate |
| Lowered by | Mixing in RO or distilled water | Mixing in RO or distilled water; active soil |
dGH, dKH and ppm: converting the units
Test kits report hardness in different units. Liquid drop kits such as the API GH and KH kit count one drop per degree (dGH or dKH) in the standard sample; strips and some charts use ppm (or mg/L) as calcium carbonate. To convert:
- 1 degree = about 17.9 ppm (more precisely 17.86 ppm as CaCO3).
- ppm ÷ 17.9 = degrees; degrees × 17.9 = ppm.
| Degrees (dGH / dKH) | Approx. ppm (as CaCO3) |
|---|---|
| 1 | 18 |
| 2 | 36 |
| 4 | 71 |
| 6 | 107 |
| 8 | 143 |
| 10 | 179 |
| 15 | 268 |
GH and KH in ppm are not the same as TDS. A TDS meter estimates everything dissolved, including sodium, nitrate and fertilizer, so it cannot tell you GH or KH. Use it as a quick stability check once you know your GH and KH (see TDS explained). Our test kit guide covers drop kits, strips and TDS meters.
Commonly cited ranges
| Shrimp | GH | KH | pH (rough) |
|---|---|---|---|
| Cherry and other Neocaridina | About 6–8 dGH | About 2–6 dKH | About 6.5–8 |
| Crystal, bee and other Caridina | About 4–6 dGH | About 0–1 dKH | About 5.8–6.6 |
| Amano and ghost shrimp | Tolerant of a fairly wide range; stability matters most (see Amano and ghost shrimp care) | ||
These are hobby guidelines, and sources vary by a degree or two. Many colonies thrive in steady water just outside them. Owner reports link losses far more often to sudden change than to a reading that is slightly off. For full cherry shrimp setup, see the cherry shrimp care guide.
How to raise GH
- A shrimp GH remineralizer (often labeled GH+) adds calcium and magnesium without carbonate, so it raises GH but not KH. This is the usual choice for Caridina, and works for Neocaridina too.
- A GH/KH remineralizer raises both. Products such as SaltyShrimp Shrimp Mineral GH/KH+ are aimed at Neocaridina and other shrimp from neutral water.
- Crushed coral, aragonite or similar calcium carbonate in a filter bag dissolves slowly and raises GH, KH and pH together. It is hard to dose precisely, so test often and remove some if readings climb past your target. Not suitable for Caridina tanks.
Mix remineralizer into the new water in a bucket, not straight into the tank, and test the bucket before using it. Follow the product's own dosing directions, since products differ.
How to raise KH
- Use a GH/KH remineralizer in new water if you also need GH. This is the simplest option for most cherry shrimp keepers on RO water.
- Crushed coral or aragonite raises KH slowly along with GH.
- Baking soda (sodium bicarbonate) raises KH and pH but not GH, and it adds sodium. It is powerful and easy to overshoot, so if you use it at all, use tiny amounts in change water and test before adding it to the tank.
How to lower GH and KH
There is no safe chemical shortcut. The reliable way is dilution: mix RO or distilled water with your tap water for water changes. Mixing equal parts roughly halves GH, KH and TDS, so tap water at 14 dGH mixed 50/50 with RO lands near 7 dGH. Test the mix, adjust the ratio, and then keep using the same recipe.
- Active soil lowers KH and pH while its buffering lasts. This is how most Caridina tanks get soft, acidic water, but hard source water uses it up quickly.
- Driftwood and leaf litter release tannins and can nudge pH down slightly in soft water, but they make little difference to GH or KH.
- Avoid household softened water. Softeners swap calcium and magnesium for sodium. GH falls, but you remove the minerals shrimp need and add sodium. See can shrimp live in tap water.
- Top off evaporation with RO or distilled water, not tap. Minerals stay behind when water evaporates, so tap top-offs push GH, KH and TDS slowly upward.
Change it slowly
If your tank is far from your target, do not fix it in one go. The common advice is to move a little at each water change: adjust the new water so it is only slightly closer to your target than the tank, and repeat over several weeks. Keep each change small and matched in temperature. See shrimp tank water changes for a routine, and add new shrimp with drip acclimation, since shop water often differs from yours.
Test and track
Test your source water's GH and KH once, and again if your water supply changes. Test the tank weekly while it settles, then monthly once readings are steady. Slow drift is easy to miss without a record, so the Water Parameter Log printable keeps GH, KH, pH, TDS and temperature on one page. If shrimp are already dying, start with why are my shrimp dying.
Frequently asked questions
What is the difference between GH and KH?
GH (general hardness) measures calcium and magnesium, the minerals shrimp use to build new shells. KH (carbonate hardness) measures carbonate and bicarbonate, which buffer pH and keep it steady.
What GH and KH do cherry shrimp need?
Commonly cited ranges for cherry shrimp and other Neocaridina are about 6–8 dGH and 2–6 dKH. Steady water matters more than hitting exact numbers.
How do I convert dGH to ppm?
Multiply by about 17.9. For example, 6 dGH is about 107 ppm as calcium carbonate. To go from ppm to degrees, divide by 17.9.
How do I lower GH in a shrimp tank?
Dilute it. Mix RO or distilled water into your water-change water, test the mix, and lower GH gradually over several water changes rather than all at once.
Does KH affect pH?
Yes. KH buffers pH. Water with a few degrees of KH tends to hold pH steady near neutral or above, while water with almost no KH lets pH drop or swing more easily.
Can I use baking soda to raise KH for shrimp?
It does raise KH and pH, but it adds sodium, does nothing for GH and is easy to overdose. Many shrimp keepers prefer a GH/KH remineralizer; if you use baking soda, add tiny amounts to change water and test first.