Water hardness refers to the concentration of dissolved minerals in water, primarily calcium and magnesium (GH), but also carbonate and bicarbonate ions (KH). Some fish species thrive in softer water (eg., tetras and discus), while others need harder water (eg., cichlids and livebearers, snails and shrimp). Understanding GH (general hardness) and KH (carbonate hardness) is critical because they affect the stability and health of an aquarium's ecosystem.
Why water hardness matters
- There are two kinds of water hardness that affect aquatic life
- GH (general hardness) measures concentration of calcium and magnesium ions
- Supports skeletal structure of fish, and shrimp and snails (exoskeleton)
- Contributes to enzyme activity and osmosis regulation for both animals and plants
- GH should be 2 degrees higher than KH, GH should not be lower than KH
- KH (carbonate hardness), measures the concentration of carbonate and bicarbonate ions
- Stabilizes pH by buffering against drastic changes
- If KH is 0, the pH can fluctuate excessively, because there is no buffering of pH, hurting plants and animals and algae will come
- Plant melting is a good indication of low KH
- KH should be 2 degrees lower than GH, KH should not be higher than GH
- Why the capacity of KH to buffer pH matters
- A bucket of rainwater at 7.0 pH will turn to 6.0 pH in a few hours, because water pH is not inherently stable, but tends toward acidity on its own
- Minerals in the water (carbonate and bicarbonate KH) buffer the pH and stabilize it, to prevent the water from becoming acidic
- Depending on the pH of the water, ammonia may be either more or less poisonous to fish
- A fish in water at pH 6.0, and you add 1 ppm of ammonia, the ammonia will not harm the fish, but will turn into ammonium and evaporate off, holds true for any pH up to 6.4
- A fish in water at pH 8.0, and you add 1 ppm of ammonia, the ammonia will be 100x more poisonous, deadly, than it would be at pH 6.0, because ammonia from 6.4 to 7.0 is 10x more powerful, the scale is logarithmic, going up exponentially
- So, pH and water hardness are very important factors for dealing with fish excretion and elimination in aquarium water
- Below pH 6.2 normal aerobic beneficial bacteria, for breaking down ammonia and nitrite (the nitrogen cycle), can no longer function properly
Demystifying KH and dKH, GH and dGH
- Scales/units for measuring carbonate hardness or general hardness
- KH (Carbonate Hardness)
- Measures total dissolved carbonates/bicarbonates
- KH acts as a pH buffer, neutralizing acids and preventing large, rapid pH fluctuations vital for minimizing stress of fish and plants
- KH determines water's resistance to pH changes, buffering capacity, low KH means unstable pH, high KH means stable pH
- Bacteria that break down ammonia and nitrite require a small amount of KH in the water to function properly
- GH (General Hardness)
- Measures total dissolved minerals (mainly calcium and magnesium) vital for fish and plants
- GH is about mineral content affecting organisms' osmotic balance, proper cell volume and function
- GH can be important for fish bone or shell development (snails, shrimp), plants need it for healthy cell walls and nutrient uptake
- Sources that raise GH (eg., limestone) often raise KH because they contain calcium carbonate, but they can be raised separately, note that low KH can allow a dangerous pH drop, even if GH is high
- dKH (degrees of Carbonate Hardness) and KH refer to the same thing
- dKH is an early German scale, KH is stated in ppm (parts per million or milligrams per liter)
- 1 dKH = 17.9 ppm
- dGH (degrees of General Hardness) and GH refer to the same thing
- dGH is an early German scale, GH is stated in ppm (parts per million or milligrams per liter)
- 1 dGH = 17.9 ppm
- The name Carbonate Hardness (KH) can be misleading
- Despite "hardness" in the name it's not the same as General Hardness (GH)
- The "K" referring to "carbonate" stems from the German word for "carbonate"
Hard Water Classifications
- Soft: 0 - 17.1 ppm (GH), 0 - 1 dGH
- Slightly hard: 17.1 - 60 ppm (GH), 1 - 3.4 dGH
- Moderately hard: 60 - 120 ppm (GH), 3.4 - 6.7 dGH
- Hard: 120 - 180 ppm (GH), 6.7 - 10 dGH
- Very hard: 180+ ppm (GH), 10+ dGH
Understanding pH (per or potential of hydrogen)
- pH measures how acidic or basic/alkaline water is, according to the concentration of hydrogen ions (positive cations) in the solution (water column)
- More hydrogen ions, more acidic, more active the solution
- Less hydrogen ions, less acidic/more alkaline, less active the solution
- The pH scale ranges from 0 to 14
- Neutral (7.0) is the center of the scale
- 0.0 is battery acid
- 2.0 is lemon
- 4.0 is tomato
- 6.0 is milk
- 8.0 is blood
- 9.0 is baking soda
- 11.0 is ammonia solution
- 13.0 is bleach
- 14.0 is drain cleaner
- For most aquariums the normal pH range will be between 6.6 and 8.0
- The pH scale is logarithmic
- Every unit on the scale is a factor of 10
- Consider 7.0 as 1
- 6.0 is 10x more acidic, 5.0 is 100x more acidic, 4.0 is 1000x more acidic, etc
- 8.0 is 10x more alkaline, 9.0 is 100x more alkaline, 10.0 is 1000x more alkaline, etc
- Therefore, the farther pH gets away from 7.0 (neutral) the larger the impact those differences will have on aquatic life health
- Every fish and every plant has its own pH sweet spot, usually somewhere between 6.0 and 8.0
- pH affects concentrations of CO2 in the water column
- Lowering pH (acidifying the water) increases the concentration of dissolved CO2
- Raising the pH (making it more basic) reduces the concentration of dissolved CO2 by converting it into carbonate or bicarbonate
- CO2 reacts with water to form carbonic acid (H2CO3), which lowers pH by releasing hydrogen cations
- Conversely, reducing CO2 (eg., through aeration or photosynthesis) raises the pH
Stabilize pH via GH and KH with respect to CO2 injection
- KH acts to stabilize pH, serving as a buffering agent
- Hydrogen ions (positively charged cations which are responsible for acid), and carbonate/bicarbonate ions (negatively charged anions which are responsible for alkalinity) are eager to bond with each other
- When the hydrogen and carbonate ions bond, they cancel each others' charges out, and that's why KH is considered a buffering component for the water column, resisting large fluctuations in pH
- Water with high KH can absorb more CO2 before the pH drops significantly, whereas water with low KH is prone to rapid pH changes when CO2 is added
- CO2 reacts with water to form carbonic acid (H2CO3), which lowers pH by releasing hydrogen cations
- Hydrogen ions (responsible for low pH--acid) bond with the carbonate and bicarbonate ions (responsible for high KH) and form carbonic acid (very weak acid compared to hydrogen ions)
- So, higher KH prevents aquarium water from going too acidic and dropping the pH
- With an adequate KH level in the water column, pH will remain stable, which is essential for the health of aquatic life
- Should we add a lot of KH to stabilize the tank pH? No, that could lead to spiking pH too high, so make changes gradually over time
- Why does pH go down?
- The carbonate cycle in aquariums involves carbonate hardness (KH) (alkalinity) buffering pH
- When organic matter decays it produces ammonia, which beneficial bacteria breaks down in the nitrogen cycle
- In the nitrogen cycle beneficial bacteria consume carbonates while converting ammonia to nitrite to nitrate, a process that releases positive hydrogen ions, cations (acid), lowering pH
- Also, CO2 itself turns into carbonic acid in water, lowering pH
- Maintaining adequate KH, typically by adding baking soda (sodium bicarbonate), replacing the carbonates consumed by bacteria in the nitrogen cycle, is crucial for stable pH and healthy bacteria
- Why does pH go up?
- Not a result of organic processes
- Carbonate and bicarbonate ions are released from crushed coral, limestone, cuttlebone, etc, which means adding calcium and mineral content which means raising GH, note that GH is not directly related to pH, but is indirectly related
- Adding baking soda (sodium bicarbonate) to the water column will also raise pH as well as KH
Desired GH and KH levels in each tank depend on the kind of fish and plants inhabiting the tank
- For a tank focused on plants GH and KH of 4 degrees is common
- Most tropical fish do well with GH and KH of 4 degrees
- However, neocaridina shrimp and ramshorn snails prefer higher GH (about 6 to 8 degrees), and plants and other tank inhabitants can adapt to that higher level
- GH should be 2 degrees higher than KH, GH should not be lower than KH
- KH should be 2 degrees lower than GH, KH should not be higher than GH
Tank pH tends toward lowering pH, especially when CO2 is injected
Raising GH and Raising KH
- Tank GH can be raised with Seachem Equilibrium (potassium, calcium, magnesium, iron, manganese), 1/4 tsp per 5 gallons raises GH by 1 degree
- Tank KH can be raised with baking soda (without changing GH but raising pH), 1/4 tsp per 5 gallons raises KH by 1 degree
- Play around, add a small amount of Equilibrium or baking soda, then test the water for GH or KH accordingly, find sweet spot gradually (starting mineral content and tank volume influence the amount needed)
- Example product - Salty Shrimp Mineral GH/KH is simply like baking soda and Equilibrium mixed together in a predetermined ratio, but it's better to have individual control over GH and KH in case you need a ratio other than what's been pre-packaged in any product
Monitor water parameters regularly, Temperature, pH, Ammonia, Nitrite, Nitrate, GH, KH
25% weekly water changes are helpful
Lowering pH, GH, KH is harder than raising -- for very hard water use RO or distilled water and add desired minerals