Guide
What to do if your water is too hard
GH and KH explained, when it matters, and how to soften water or choose fish that suit your tap.
- GH vs KH: GH (general hardness) measures calcium and magnesium; KH (carbonate hardness) measures buffering capacity. Both matter, but for different reasons.
- Test first: "Hard water" covers a wide range - 150 ppm and 450 ppm are both technically hard, but they're not the same problem. Get a number before doing anything.
- Species matching beats water chemistry battles: If your tap runs consistently hard, choosing fish that thrive in hard water is almost always easier and safer than fighting your supply.
- To soften safely: Blend your tap water with RO or distilled water at water changes. Peat and botanicals help but work slowly and inconsistently.
- Never soften suddenly: A large, rapid drop in GH or KH stresses fish and can crash your pH. Gradual change over days or weeks is the rule.
- Snails and shrimp need calcium: Don't soften so aggressively that GH drops below ~4-6 °dH - inverts need dissolved minerals for shells and exoskeletons.
What GH and KH actually measure
Hard water is tap water with a high concentration of dissolved minerals - mainly calcium (Ca²⁺) and magnesium (Mg²⁺). These are what your GH (general hardness) test measures. The unit you'll see most often is degrees of hardness: °dH (German degrees) or ppm (parts per million). The conversion is simple: 1 °dH = 17.9 ppm.
Most people's confusion starts here: there's a second hardness reading called KH (carbonate hardness), sometimes labelled alkalinity. KH measures bicarbonate and carbonate ions - the chemicals that resist pH swings by absorbing acid. A high KH acts like a chemical buffer; a low KH means your pH is unstable and vulnerable to crash. Hard tap water typically has both high GH and high KH, but not always - you can have one without the other.
A rough guide to GH ranges:
- 0-4 °dH (0-70 ppm): Soft - typical of rainforest regions, Scottish/Scandinavian supply areas
- 5-12 °dH (90-210 ppm): Moderately hard - suits a wide range of fish
- 13-20 °dH (230-360 ppm): Hard - ideal for livebearers, African cichlids, hard-water tetras
- 20+ °dH (360+ ppm): Very hard - most of southern England, parts of the US Midwest and Southwest
If you haven't tested your water yet, a liquid test kit for GH and KH is the first step. Strip tests give a ballpark; liquid reagent kits (API, Salifert) are more reliable. Log the results and track them over time - municipal supply hardness can shift seasonally, and App-aquatic makes it easy to keep a running record of your readings alongside other parameters.
Your water company publishes annual water quality reports that include hardness figures for your area. Look up your postcode or zip code on their website - it's a fast free sanity check before you buy a test kit, though it won't tell you if your building's plumbing adds anything.
When hard water is actually a problem
Hard water by itself doesn't kill fish. Most commercially-bred fish in the hobby have been raised in moderately hard water for generations, regardless of their wild origins. The real problems arise in three specific situations.
You're keeping fish that genuinely need soft water
A handful of species do poorly long-term in hard water - not because hard water is toxic, but because it shifts pH in a direction that doesn't suit them and may affect reproduction, colour, and immune response. The species that genuinely need soft, acidic conditions include:
- Wild-caught discus (Symphysodon spp.) and altum angelfish (Pterophyllum altum)
- Cardinal tetras (Paracheirodon axelrodi) - note that common/neon tetras are far more adaptable
- Chocolate gouramis (Sphaerichthys osphromenoides)
- Some Apistogramma dwarf cichlids, particularly wild-caught specimens
- Many wild-caught Southeast Asian rasboras
Tank-bred versions of most of these are noticeably more tolerant. A tank-bred cardinal tetra at a GH of 10 °dH is unlikely to suffer. A wild-caught discus at 15 °dH will.
Your KH is so high it locks your pH at an unfavourable level
Very hard water often comes with a KH of 15+ °dH, which pins pH firmly above 7.8-8.0. For fish that prefer neutral to slightly acidic conditions (pH 6.5-7.2), this is harder to work around than the GH alone. The buffer is actively fighting your attempts to acidify the water.
Snails, shrimp, or crayfish are showing shell problems
This is the reverse of most hardness complaints. If you've been softening your water aggressively, calcium-dependent invertebrates will show it: pitted snail shells, thin or cracked shells in mystery snails (Pomacea bridgesii), and failed molts in shrimp. Keep GH above 6 °dH (roughly 100 ppm) if you're keeping any inverts.
Most online guides make hard water sound like an emergency that needs fixing immediately. It usually isn't. If your fish are eating, behaving normally, and not showing disease, your water is probably fine for the species you have. The problems that hard water causes are almost always slow and chronic - not overnight crises. You have time to make changes gradually and correctly.
The easiest solution: match your fish to your water
Before you invest in an RO unit or start hauling distilled water home from the supermarket, consider stocking fish that actually prefer hard water. This isn't a compromise - it's just good fishkeeping. These species will be healthier, colour up better, and breed more readily in your tap water than most soft-water fish ever would:
- Livebearers: Guppies (Poecilia reticulata), mollies (Poecilia sphenops), platies (Xiphophorus maculatus), and swordtails (X. hellerii) all thrive at GH 12-25 °dH, pH 7.2-8.2
- African Rift Valley cichlids: Malawi and Tanganyika cichlids demand hard, alkaline water - GH 12-20 °dH, KH 10-20 °dH, pH 7.8-8.5. This is their element.
- Rainbowfish: Most Melanotaenia species do well at GH 8-15 °dH
- Many Central American cichlids: Convicts (Amatitlania nigrofasciata), firemouths (Thorichthys meeki), and severums (Heros efasciatus) tolerate hard water well
- Common tetras: Ember tetras, black skirt tetras, and X-ray tetras are all more hard-water tolerant than their reputation suggests
Check our best fish for beginners guide for a wider list of adaptable species, and use the free stocking calculator to check compatibility before you buy.
How to safely soften hard water
If you've decided that softening is the right move - because you're committed to a specific species group or you want to breed soft-water fish - here's how to do it without causing more problems than you solve.
RO water: the reliable method
Reverse osmosis (RO) filtration strips virtually all minerals from tap water, giving you a blank slate you can adjust precisely. The process:
- Produce RO water (either from a home unit or bought from an aquarium shop)
- Mix with your tap water at water changes to hit your target GH. A 50/50 blend of very hard tap (say, 20 °dH) and RO will land around 10 °dH - test to confirm
- If you go heavy on RO and KH drops below 3-4 °dH, add a small amount of sodium bicarbonate or a commercial KH buffer to prevent pH instability
A home RO unit costs £80-£150 / $100-$200 and pays for itself quickly if you're doing regular water changes on multiple tanks. The main running cost is wasted water - RO systems typically produce 3-4 litres of waste water for every litre of RO output. See our guide on how often to do water changes to work out how much water you'll realistically need.
Pure RO water has a GH and KH of nearly zero. Adding it directly to a tank without blending - especially in large volumes - can cause a rapid drop in pH and osmotic stress to fish. Always pre-mix your RO blend in a bucket before it goes in the tank, and never do a 100% water change with undiluted RO water.
Distilled water: same idea, higher cost
Distilled water behaves identically to RO for aquarium purposes. It's fine for small tanks or occasional top-offs, but buying it in 5-litre bottles gets expensive fast for anything larger than 60 litres.
Peat filtration
Adding dried peat (unfertilised, compressed peat moss - not garden peat with added nutrients) to a filter bag in your filter will release humic and tannic acids that gradually lower GH, KH, and pH while tinting the water amber. This mimics blackwater conditions and suits fish like Apistogramma, wild bettas, and many tetras.
The catches: peat works slowly, the effect is hard to quantify precisely, and the tannin staining isn't to everyone's taste. It's a decent supporting measure but not a precise tool. Replace the peat every 4-6 weeks as it exhausts.
Indian almond leaves and driftwood
Both release tannins and humic acids that provide mild acidifying and softening effects. Indian almond leaves (Terminalia catappa) are more potent per gram; driftwood works more slowly. Neither will make a dramatic dent in very hard water - think of them as fine-tuning tools or aesthetic additions that happen to help, not as primary softeners.
Softening water by adding a commercial "pH Down" product (usually phosphoric acid) without also addressing KH. In hard water with high KH, the buffer immediately neutralises the acid, so you end up adding large amounts of acid, the pH briefly drops, bounces back, and you add more. The result is chemical instability that stresses fish far more than the original hardness did. If you want lower pH, reduce KH first - then pH becomes much easier to manage.
Managing the change: avoiding dangerous swings
Fish can adapt to a fairly wide range of water hardness - what they can't handle is rapid change. The osmotic stress of a sudden large shift in GH or KH can trigger disease, white-spot outbreaks, and in extreme cases death.
The rule of thumb: change GH and KH by no more than 1-2 °dH per day. In practice, this means:
- Start blending RO at a modest ratio - say 20% RO, 80% tap - at your first few water changes
- Test after each change and move gradually toward your target over several weeks
- Once you're at your target mix, be consistent - always use the same ratio so the water parameters in the tank stay stable
This is especially important if you're moving fish from your existing hard-water setup into a newly softened tank. Acclimate them slowly using the drip method, or float-and-add over at least 30-45 minutes.
Consistency matters as much as the actual number. A fish living in 12 °dH water that never fluctuates is healthier than a fish in 8 °dH water that yo-yos between 6 and 10 °dH every water change. For more on what stable parameters look like and why the nitrogen cycle interacts with water chemistry, see our aquarium water parameters guide.
Keeping partial water changes at or below 30% gives you finer control over parameter shifts and reduces osmotic stress during the transition period.
What about water softener salt?
Household water softeners work by ion exchange - they swap calcium and magnesium ions for sodium ions. The result is water that feels soft and doesn't scale your kettle, but still has high TDS (total dissolved solids) and elevated sodium. This water is not suitable for fish tanks. Sodium at high concentrations is harmful to freshwater fish, and the ion exchange process doesn't remove it - it adds it. Never use water from a household ion-exchange softener in an aquarium.
Cycling a new soft-water tank
One thing that catches people out: if you're setting up a new tank with soft, low-KH water, the cycling process can be less stable than in harder water. Nitrifying bacteria produce acids as a byproduct of the nitrogen cycle, and in low-KH water there's less buffering to absorb this. pH can drift downward during cycling. Test KH and pH regularly during the cycle - if pH drops below 6.5, add a small amount of baking soda to bring KH back up, then let the cycle continue.
Track your GH, KH, and pH readings over time, set parameter alerts, and manage water change schedules with App-aquatic.
Get the free appMy GH is 20 °dH. Can I keep tetras?
Many tetras, yes. Black sk
More guides · Water parameters · pH too acid or alkaline · Tap water · Get the app