Aquarium Water Hardness: GH, KH, and Why They Matter

Hardness preferences fall into three rough categories.

Soft-water fish (low GH, low to moderate KH): discus, most wild-type tetras including cardinals, German blue rams, apistogrammas, chocolate gouramis, and most Southeast Asian rasboras. These fish evolved in blackwater rivers where mineral content is extremely low, with GH of 1-4 dGH and sometimes near-zero KH. In hard tap water they often survive but rarely breed and are more susceptible to disease.

Neutral to moderate hardness (GH 6-12 dGH, KH 4-8 dKH): most community fish fall here, including bettas, guppies, platies, mollies, danios, corydoras, most barbs, and the majority of cichlids not from the African rift lakes. This range suits tap water in most cities without adjustment.

Hard-water fish (high GH, high KH): African rift lake cichlids (peacocks, aulonocara, mbuna, tanganykians) need GH above 10 dGH and KH above 10 dKH with pH in the 7.8-9.0 range. Livebearers like mollies and swordtails also prefer moderately hard water, though they tolerate a wider range.

Shrimp are a special case. Neocaridina shrimp (cherry shrimp, blue dreams, red rilis) prefer GH of 6-8 dGH and KH of 2-4 dKH. Caridina shrimp (crystal reds, bees, tigers) need much softer water: GH of 4-6 dGH and KH near 0-2 dKH with specific remineralization products.

If your tap water is soft and you need to raise GH for hard-water species or to hit the remineralization target for neocaridina shrimp, a few methods work reliably.

Seachem Equilibrium is the standard planted tank and shrimp keeper solution. It adds calcium, magnesium, and potassium without affecting KH or pH significantly. Dose according to the target GH on the label; roughly 1 teaspoon per 10 gallons raises GH by about 3 dGH depending on starting values. Add to new water before putting it in the tank.

SaltyShrimp GH/KH+ is specifically formulated for neocaridina shrimp tanks built on RO or very soft water. It raises both GH and KH to the ratios neocaridina prefer. Widely used among shrimp keepers because it hits both parameters consistently from near-zero RO water.

Crushed coral and aragonite in the filter or substrate slowly release calcium and magnesium, raising both GH and KH over time. The release rate is pH-dependent: more dissolves when pH is lower, less when pH is stable. This makes it self-regulating for KH (see next section) but less precise for GH targeting.

Hard tap water blended with RO water is the easiest method when your goal is moderate GH. If tap water tests at 15 dGH and you want 8 dGH, a 50/50 blend lands you close. Test the blend before adding fish.

Low KH is the hidden cause of the pH crashes that confuse a lot of fishkeepers. The pattern looks like this: tank runs at pH 7.4, then slowly drifts down to 6.8, then 6.5, fish get stressed. What happened is the KH buffer got consumed by organic acids produced from fish waste and plant respiration. Once KH hits zero, pH free-falls.

If your KH is below 3 dKH, raising it is the fix. A few approaches:

Crushed coral in a mesh bag in the filter is the most controlled method. Calcium carbonate dissolves slowly as pH drops, releasing carbonates that both stabilize pH and raise KH. Add a cup of crushed coral to your filter media and let it work over a week. Test KH at 3-day intervals. Easy to adjust by adding or removing coral. CaribSea Crushed Coral or aragonite both work well.

Baking soda raises KH directly and quickly. Sodium bicarbonate is cheap and reliable; 1 teaspoon per 10 gallons raises KH roughly 4 dKH. Dissolve it fully in a cup of tank water before adding. The downside is it also raises pH somewhat and adds sodium, which accumulates over time in tanks with frequent dosing. Good for one-time corrections, less ideal for ongoing maintenance.

Seachem Alkalinity Buffer or similar commercial KH buffers raise carbonate hardness without the sodium load of baking soda. More expensive per dose but cleaner for ongoing use.

The goal for most community tanks is KH in the 4-8 dKH range. This provides enough buffering to absorb daily CO2 fluctuation without allowing large pH swings.

Hard tap water creates real challenges for discus, wild-type tetras, German blue rams, and other soft-water fish. The standard solution is RO water.

Reverse osmosis filtration removes virtually all dissolved minerals, producing water with near-zero GH and KH. You then blend RO water with tap water to hit your target hardness, or use a remineralization product to add back only what you want. Discus breeders typically run 100% RO with a small amount of remineralizer to keep GH around 2-4 dGH. Community soft-water fish can often tolerate a 50/50 blend with most tap water.

Peat moss in the filter lowers both GH and KH over time through ion exchange and humic acid release. It also stains the water amber, which soft-water fish from blackwater rivers actually prefer. Neons and cardinals look better under tannin-stained water. Effects are slow and dose-dependent; use a mesh bag and test weekly to gauge how much is needed.

Rainwater collection is a free source of very soft water if you have a clean collection setup away from treated roofs. It needs testing for pH and treatment with a water conditioner for any trace contaminants before use.

Trying to use chemical softeners (like water softener salt or certain commercial products) does not actually lower GH the way fish need. Ion exchange water softeners replace calcium and magnesium with sodium, which is not helpful for soft-water fish and can be harmful to shrimp. Avoid using home water softener output for aquariums.