Aquarium water hardness: GH, KH and why they're different

What GH and KH actually measure

GH (general hardness) is, in scientific terms, the total dissolved calcium and magnesium in water[1]. These minerals dissolve into groundwater as it passes through soil and rock, so how hard your tap water is depends largely on the geology your water supply comes from[1] — it isn't something a water company adds deliberately.

KH (carbonate hardness), more precisely called alkalinity, measures dissolved carbonate and bicarbonate instead — the water's capacity to neutralise acids and resist a change in pH[2]. Water with higher KH buffers against pH swings and recovers more easily after a disturbance than water with low KH, which can swing further and faster for the same input[2]. See our aquarium pH guide for the full explanation of that relationship.

Why they're not the same thing

Both are traditionally reported using the same "degrees" unit and both commonly come from the same calcium-carbonate-related chemistry, which is exactly why they get treated as one reading. But they measure genuinely different dissolved substances, and a water supply can carry them independently. Rainwater and reverse-osmosis water are low in both. Many limestone-influenced tap supplies are moderately high in both together. But it's entirely possible to have water that's relatively soft in GH while still carrying meaningful KH, or the reverse — particularly once a tank's own substrate, décor, or additives start influencing one more than the other. Knowing which one is actually out of range, rather than assuming "hardness" is a single problem, is the difference between an effective fix and a wasted one.

Units and testing

This is one of the more genuinely confusing corners of the hobby, because both readings get reported in at least three different units depending on your test kit or region: degrees (dGH/dKH, sometimes written °GH/°KH), parts per million or milligrams per litre as calcium carbonate (ppm/mg/L CaCO3), and occasionally millimoles per litre (mmol/L). One degree of hardness on the German scale — used for both GH and KH — equals approximately 17.85 mg/L as CaCO3, a fixed unit-conversion constant rather than an aquarium-specific finding. Check exactly what your test kit or liquid reagent reports before comparing your reading against any published target, since a figure that looks alarmingly high in one unit can be perfectly unremarkable in another.

Hobby test kits typically use either a liquid reagent with a drop-count titration (count how many drops cause a colour change, then multiply by a factor the kit specifies) or test strips. Titration kits are generally considered more precise, but are also easier to misread if drops aren't fully mixed in between, or if the colour-change point is judged inconsistently — retest if a result looks unexpected before acting on it.

Why GH matters

Calcium and magnesium aren't just a passive background reading. Many freshwater invertebrates — snails and shrimp in particular — need adequate dissolved calcium to build and maintain their shells or exoskeletons; see our mystery snail, blueberry snail and cherry shrimp guides for species where this is a genuine, recurring welfare consideration rather than a minor detail. Aquatic plants also draw on calcium and magnesium as nutrients, so very soft water can limit plant growth alongside any livestock effect. On the fish side, many species have adapted over a long evolutionary history to a fairly narrow natural GH range — soft, mineral-poor blackwater species from the Amazon basin at one extreme, hard-water rift-lake cichlids at the other — and keeping a species well outside the range it's adapted to is a recognised source of chronic stress, even when the fish doesn't show an obvious acute symptom.

Why KH matters

KH's role is less about direct physiology and more about stability. A tank with adequate KH resists the gradual pH drift that biological processes cause over time — nitrification itself consumes alkalinity as it proceeds, so a tank that starts with low KH can see its pH creep downward between water changes even with nothing else obviously wrong. A tank with very low KH is also more vulnerable to a sudden pH crash from a comparatively small trigger, since there's little buffering capacity left to absorb it. This is why KH is often the more urgent of the two readings to know, even though GH tends to get more attention in care-sheet stocking tables.

Typical ranges by tank type

These reflect widely repeated hobby conventions gathered from specialist sources and species-specific requirements published elsewhere on AquaPulse, not a single scientific standard that applies everywhere. Treat them as a starting orientation, and defer to your specific species' documented needs over a generic category.

Marine and reef tanks use a related but distinct set of parameters — alkalinity, calcium and magnesium measured on their own reef-specific target ranges rather than freshwater GH/KH conventions. See our Reef Dosing Calculator if that's what you're working with.

Raising or lowering GH

To raise GH: crushed coral, aragonite substrate, or a remineralising mineral salt product formulated for aquarium use are all commonly used. Crushed coral and aragonite dissolve gradually, so they raise hardness slowly and continuously rather than all at once, which many keepers find easier to manage than a single large dose. A remineralising salt product is the standard way to prepare reverse-osmosis or deionised (RO/DI) water for use, since RO/DI water on its own has essentially no minerals left in it and isn't suitable for most livestock without them added back.

To lower GH: blending in RO/DI water with your tap water, or sourcing naturally soft water, are the usual approaches. Driftwood and certain other natural décor can contribute a mild softening and acidifying effect over time through released tannins, though this is a slow, secondary effect rather than a reliable primary method for a significant adjustment. Our RO/DI Water Mixing Calculator works out the actual blend ratio from your own tap and target readings, rather than leaving you to estimate it.

Raising or lowering KH

To raise KH: sodium bicarbonate (ordinary baking soda) is a widely used, inexpensive way to raise KH specifically, since it adds bicarbonate without contributing meaningful calcium or magnesium — useful if you want more buffering without also pushing GH up. Crushed coral and aragonite raise KH as well as GH, since the same calcium carbonate dissolution contributes to both readings together.

To lower KH: RO/DI water blended with tap water is the most controllable method. Peat filtration and driftwood can also reduce KH gradually as part of a broader soft-water, acidic-leaning setup, though again as a slow, secondary contributor rather than a precise, immediate adjustment.

What to do

Verify

1. 1

   Test your source water for both GH and KH

   Establish your realistic starting point before assuming either needs correcting, and note which unit your kit reports.

2. 2

   Check what your actual livestock needs

   Look up your specific species rather than a generic "freshwater" target — needs genuinely vary, sometimes considerably, between species kept in the same general category of tank.

If adjustment genuinely seems warranted

1. 3

   Identify which reading actually needs changing

   Don't assume "hardness" is one problem — confirm whether it's GH, KH, or both that's out of range for your livestock, since the fix differs.

2. 4

   Adjust gradually and retest

   Whichever method you use, change it over days rather than in one large step, and retest between adjustments — livestock generally tolerate a slow shift far better than an abrupt one, even toward a "correct" number.

What not to do

• Don't assume GH and KH are the same reading, or that fixing one automatically fixes the other.
• Don't make a large, sudden hardness adjustment, even toward a species' published "ideal" figure.
• Don't add crushed coral or aragonite expecting to raise GH without also raising KH and pH — the three move together with this method.
• Don't use RO/DI water without remineralising it first; unadjusted RO/DI water is unsuitable for most livestock.
• Don't chase a species' published "ideal" hardness if your livestock are already stable and healthy at a different, consistent value.

Prevention

• Test your source water's GH and KH once, so you have a realistic baseline before choosing livestock or planning any adjustment.
• Choose species whose needs are already reasonably close to your tap water's natural hardness where practical, rather than committing to ongoing correction.
• Keep a consistent water-change routine using appropriately matched replacement water — see our guide to aquarium water changes.
• If using RO/DI water, remineralise it consistently to the same target every time, rather than varying it batch to batch.
• Monitor KH alongside pH, since low KH is what allows pH to swing significantly in the first place — see our pH guide.

Frequently asked questions

How AquaPulse Water Check can help

GH and KH are easy to test and oddly hard to interpret afterwards, because what counts as normal isn't universal — it's specific to your source water, your substrate, and whatever's living in the tank. The only way to know if today's reading is a genuine change is to have a previous one to put next to it. That's the part AquaPulse handles: your GH and KH logged together over time, so a shift after a new bag of substrate or a change in tap water shows up as exactly that, instead of an unexplained number with nothing to compare it to.

Water Check works from the information you enter — it doesn't take automatic readings and isn't a substitute for testing your water with a reliable kit, or for professional or veterinary advice when livestock are seriously or persistently unwell. Get AquaPulse on the App Store or Google Play to try it, or see the Water Check feature page for more detail.

Sources and editorial information

Good to know: This guide provides general informational guidance only. It is not a veterinary service and does not replace specialist or aquatic health advice for serious livestock health issues.

• [1] US Geological Survey — "Hardness of Water" Used for the definition of general hardness (dissolved calcium and magnesium), its geological origin, and the general water-supply classification scale (soft/moderately hard/hard/very hard). A general water-quality classification, not an aquarium-specific target.
• [2] US Geological Survey — "Alkalinity and Water" Used for the definition of alkalinity (KH), its carbonate/bicarbonate basis, and its buffering role. Also cited in our pH guide for the same underlying chemistry.
• [3] Merck Veterinary Manual — "Environmental Diseases of Aquatic Animals in Aquatic Systems" Used for the general principle that livestock kept well outside their adapted water-chemistry range experience chronic physiological stress, consistent with this source's use elsewhere on AquaPulse.

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