Water Chemistry Guide For Healthy Fish Tanks

As fish keepers, we are told that it is important to pay attention to water chemistry by measuring the pH, kH, and gH of our aquariums and source water. Fish suitability is commonly defined in terms of the pH and gH of their natural habitat. Many aquarists, even though they measure these perimeters, don’t completely understand them, know how they can interact, or the effect they may have on fish. The purpose of this article is to try and provide some definition and clarity to the terms, determine how they interact as well as their effects upon tropical fish.

Water Chemistry – pH

pH Definition

pH comes from the French word hydrogène, and means ‘hydrogen power’. It equates to the amount of hydrogen (H+) and hydroxide (OH-) ions are dissolved in a solution. The more hydrogen ions there are, the more acidic the water is and the lower the pH is. A solution that has equal concentrations of hydroxide and hydrogen is termed neutral with a pH value of 7. A higher concentration of hydroxide ions would return a value above 7 or alkaline. A higher concentration of hydrogen ions would return a value below 7 or acidic. The pH scale is logarithmic, in other words, each step up or down is 10 times that of the previous one. A pH of 6 is ten times more acidic than a pH of 7. A pH of 5 is 100 times more acidic than 7 and so on.

Most freshwater fish live within a pH range of 5.5 to 7.5 (African chiclids can go up to 8.4). Since the scale is logarithmic, this range represents a variation of over a 1000 times. Even an apparently small change in pH can affect fish, causing stress or death.

Water Chemistry Guide For Healthy Fish Tanks

Impacts of pH on Fish

The consequences for fish are many and varied. It affects their breathing ability. High acidity or alkalinity can cause direct physical damage to skin, gills and eyes. Prolonged exposure to sub-lethal pH levels can cause stress, increase mucus production and encourage epithelial hyperplasia (thickening of the skin or gill epithelia) with sometimes-fatal consequences.

There are indirect consequences that can also affect fish. Changes in pH will affect the toxicity of many dissolved compounds. For example, ammonia becomes more toxic as pH increases. Fluctuations in pH, even though they may still be within the preferred range, can be stressful and damaging to fish health.

Nitrifying bacteria, essential in the conversion of ammonia to nitrate, also have a pH range preference, which is between 7.5 and 8.6. Variations in pH will also have an effect on some disease treatments. Chloramine-T is more toxic at low pH, while potassium permanganate is more dangerous at high pH.

Monitoring the pH in an established aquarium can often indicate water change and substrate vacuuming needs, or a clogged UGF. Excess waste product produces carbonic acid, which acidifies the water and lowers the pH./h2.

While selecting fish that are compatible to the pH of the water used to fill the aquarium is the best method and avoids the need to change the pH, many aquarists want keep a species of fish that may require pH alteration. Many fish can accept a limited pH range, however breeding may be more difficult if not impossible. There are methods of altering the pH in your aquarium.

Ways to lower pH

  • Filter water over peat
  • Add bogwood to the tank
  • Inject carbon dioxide CO2
  • Use a commercial acid buffer
  • Water changes with softened water or RO (Reverse Osmosis) water

Ways to raise the pH

  • Aerate the water, driving off the carbon dioxide (CO2)
  • Filter over coral or limestone
  • Add rocks containing limestone to the tank or use a coral sand substrate
  • Use a commercial alkaline buffer

Any changes to pH should be done gradually!!!

Water that is poorly buffered (low kH or temporary hardness) will be subject to higher pH fluctuations than well-buffered water. As a general rule, hard water is usually alkaline (above 7) and well buffered, whereas soft water (below 7) is usually slightly acidic and poorly buffered. Permanent hardness also has an effect on the pH therefore in order to completely understand pH, we must also understand water hardness.

Water Chemistry – Water Hardness

Water Chemistry Guide For Healthy Fish Tanks

Water Hardness Definition

Water accumulates many dissolved substances before it reaches our taps. Hardness is a measurement of the concentration of metal ions such as calcium, magnesium, iron, and zinc. Most of these concentrations are acquired as rainwater passes over rocks. In most water, it consists mainly of calcium and magnesium salts, with trace amounts of other metals.

Types of Water Hardness

There are two types of hardness that we need to consider. Permanent hardness and alkalinity (often referred to as carbonate or temporary hardness) (kH). The sum of both types of hardness is called general hardness (gH)

Alkalinity or temporary (carbonate) hardness (kH) refers to the hardness derived mainly from carbonate and bicarbonate ions and directly reflects the buffering capacity of the water. It can be precipitated and removed by boiling the water. This is why lime-scale forms in kettles and showerheads.

Permanent hardness measures ions such as nitrates, sulfates, and chlorides, etc, and cannot be removed by boiling.

Water Hardness & Buffering

While there is a connection between water hardness and buffering, hardness is a product of mainly calcium and magnesium ions, and buffering is produced by bicarbonate and carbonate ions. As mentioned earlier, hard water is usually well buffered and soft water is usually less buffered. It is possible though, based on different water compositions, to have hard water that is poorly buffered or soft water that is well buffered. The way to establish the makeup of your local water is by using a test kit and test for both gH (general hardness) and kH (temporary hardness).

Water Chemistry – kH

Water Chemistry Guide For Healthy Fish Tanks

kH Definition

KH is carbonate hardness or temporary hardness and it measures the buffering capacity or the ability to absorb and neutralize added acid without major changes to pH. Think of buffering capacity as a big sponge, the higher the buffering, the bigger the sponge. How much buffering does your tank need? The higher the kH (the bigger the sponge), the more resistant to pH changes your water will be. A tank’s kH should be high enough to prevent large pH swings over time. If your kH is below roughly 4.5 OdH, you should pay special attention to your tank’s pH (e.g., testing periodically) until you get a feel for how stable the pH is.

kH & Buffering

Buffering is both good and bad. On the good side, the nitrogen cycle in our tanks produces nitric acid (nitrate). If we don’t have to buffer (kH), the pH will drop over time. Sufficient buffering will keep the Ph stable. On the bad side, hard water almost always has a large buffering capacity and if the pH is too high for your fish, this large buffering capacity will make it more difficult to lower the pH.

Buffering is sometimes referred to as ‘alkalinity’ but should not be confused with ‘alkaline’. Alkalinity refers to buffering and alkaline refers to a solution that is base rather than acid (pH).

Aquariums with a low kH will require more attention to water changes to control the nitrate level reducing the tendency for the pH to drop.
As with pH, there are ways to increase and decrease the buffering capacity of your water.

Ways to increase kH

  • Adding sodium bicarbonate (baking soda). One teaspoon of baking soda added to 50 liters of water can raise the kH of the water by approx 4 OdH without a major effect on pH.
  • Adding an air stone to increase surface turbulence driving off carbon dioxide (CO2)
  • Adding commercially available products to increase buffering capacity

Ways to lower kH

  • Injecting carbon dioxide (CO2)
  • Use reverse osmosis (RO) water. You can mix tap water with reverse osmosis water to achieve the desired kH.
  • Adding commercially available products to decrease the buffering capacity.

It is not a good idea to use distilled water in your tank. By definition, distilled water has essentially no kH. That means that adding even a little bit of acid will change the pH significantly (stressing fish). Because of its instability, distilled (or any essentially pure water) is never used directly. Tap water or other salts must first be added in order to increase its gH and kH.

Water Chemistry – gH

gH Definition

General hardness (GH) refers to the dissolved concentration primarily of magnesium and calcium ions. Other ions can contribute to water hardness but are usually insignificant and difficult to measure. When fish are said to prefer “soft” or “hard” water, it is gH, not kH that is being referred to. gH will not directly affect pH although ‘hard’ water is generally alkaline due to some interaction of gH and kH.

Impacts od gH on Fish

Incorrect gH will affect the transfer of nutrients and waste products through cell membranes and can affect egg fertility, proper functioning of internal organs such as kidneys and growth. Within reason, most fish and plants can successfully adapt to local gH conditions, although breeding may be impaired.

Most test kits measure gH or general hardness in German degrees hardness or OdH, which is equal to 17.9 mg/L. Since mg/L is equal to ppm (parts per million) simply multiply the degrees OdH times 17.9 if you prefer to work with ppm. The following table will give an idea of how hard your water may be after reading the test results.

Water Chemistry Guide For Healthy Fish Tanks

Ways to increase gH

  • Adding limestone to the aquarium (this will also increase kH which in turn will increase pH)
  • Adding calcium carbonate will raise gH and kH

Ways to reduce gH

  • Adding peat moss to your filter
  • Use commercially available water softening pillows or a water softener (this removes calcium and magnesium ions and replaces them with sodium ions. Many people feels that this is an unacceptable method of softening water as many fish that prefer soft water don’t like sodium either.
  • Mixing tap water with reverse osmosis (RO) water.

It is more difficult to change gH without affecting kH than it is to change kH without affecting gH as you can see.


  • While distinct, pH, kH, and gH interact and affect each other. If you change one parameter, be sure and monitor the others to see the effect.
  • It is easiest and best to raise fish that are compatible with the water parameters you are dealt.
  • Make changes gradually.
  • When making changes it is usually best to do it in containers outside the aquarium, then add the treated water to the aquarium.
  • If you have a low kH, increase water changes accordingly and monitor pH more frequently.
  • Understand that decorations such as driftwood, bogwood, limestone, filtering with peat, etc. will affect the kH and pH of the aquarium.

Good luck and happy fishkeeping.

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