In fishkeeping, water chemistry deals with the chemical properties and information ("parameters") about water that are relavant to maintaining a fish (and other life form) friendly environment. The quality of aquarium water is determined by measuring water chemistry.


Below we will outline some of the major parameters that are monitored in an aquarium. Though listed independently, you will come to see how interconnected they are, and how much they vary with each other. 


Regarding changes in these parameters, you will also note the common theme that sudden shifts are almost always detrimental, which gives rise to our RULE TO LIVE BY:

"In aquariums, nothing good happens fast"


pH is a number that represents the acidity or alkalinity (acid or base) of a solution. 7.0 is a neutral pH, with numbers greater than 7 representing a more basic solution, and numbers lower than 7 representing a solution that is more acidic. pH is measured on a logarithmic scale ranging from 1-14, with an average freshwater aquarium falling withing the 6.5-8.0 range.


pH is of concern to fishkeepers for two reasons:

  1. Different species of fish have adapted to thrive within certain pH ranges. The pH of the water in your aquarium should be considered when choosing a type of fish as, while adjusting pH is possible with the addition of chemicals, it is difficult to perfect and maintain, which is dangerous in itself (see 2. below)
    1. Waters around the world vary greatly in their pH values, and thus fish with may different optimal pH ranges exist.
  2. Sudden or large changes in pH (anything greater than 0.4 over a few hours) are very stressful to fish and can cause great harm.
    1. If the change in pH, or resultant distress, are severe enough, pH Shock and even death can result.
    2. While most animals can adjust to most pH values, even those outside of their optimal range, they must make this adjustment incredibly gradually.
    3. For this reason, tampering with the existing pH of your own water can be very dangerous, as rapid swings can result.
      1. It is better to maintain a steady pH slightly outside of the fish's optimum range than to create rapid swings in pH while trying to obtain a different value.


For these reasons, you should test to determine the pH of your aquarium, and be prepared to re-test at any sign of distress. Changes in pH can happen, sometimes suddenly, without any input from you. For more on this, see our pH CRASH article.


KH is also known as the "carbonate hardness," "buffering capacity," "total alkalinity," or "acid-neutralizing ability" of water. It is related to pH, and refers to the water's ability to keep pH stable over time as bases and acids are added. This buffering capacity is produced by dissolved minerals in the water - mostly carbonates and bicarbonates, hence the term "carbonate hardness".


As an example, when an acidic solution is added to water with a neutral pH, that water does not automatically become more acidic as the two mix - instead the water's buffering capacity "absorbs" the added acidity like a sponge, maintaining the water's neutral pH. But, like a sponge, buffering capacity has a limit to what it can hold and can be used up. 


The end product of the nitrogen cycle, nitrate (nitric acid), is acidic and as it is constantly produced, eats away at water's buffering capacity over time, causing an aquarium's pH to gradually decrease. Regular partial water changes replace depleted water with well buffered, fresh water, maintaining kH. Without these water changes, a devastating pH CRASH would ultimately result.


GH stands for general hardness, and refers to the concentration of dissolved magnesium and calcium ions in the water. When water is referred to as being "hard" or "soft," it is GH that is being described, not KH.


Like pH, different fish species have different preferences for "hard" or "soft" water, and thus testing to determine where your water falls in this spectrum is useful.


Though all distinct parameters, GH, KH and pH interact together to varying degrees.

Ammonia (NH3/NH4)

Ammonia is a chemical seen in the aquarium as a metabolic waste product constantly produced by fish, and produced by the decay of organic matter.


Ammonia is toxic to fish when present in any amount, burning and damaging gills and skin, and causing rapid death. It is the killer in "NEW TANK SYNDROME", which you can read about on our page.


Generally, when fishkeepers talk about ammonia, they are referring to unionised, or "free" ammonia - NH3 - but ammonia exists in two forms in the aquarium:

  • NH3 - ammonia (unionised, "free")
    • extremely toxic
  • NH4 - ammonium (ionised)
    • infinitely less toxic, to the point of being disregarded


This distinction is made here to illustrate two points:

  1. Many aquarium ammonia test kits test total ammonia (NH3 and NH4);
    1. This does NOT mean that the results of an ammonia test are incorrect and any measurable ammonia must still be addressed as toxic in an aquarium.
  2. Of the total ammonia amount, the balance of NH3 and NH4 molecules shifts as pH and temperature change.
    1. In a low (more acidic) pH and/or low temperature, more of the NH4 (ammonium, relatively harmless) molecules will exist;
    2. In a high (more basic) pH and/or high temperature more the NH3 ("free" ammonia, toxic) molecules will exist.

Note that attempting to suddenly lower the ph and/or temperature of your aquarium are not presented here as solutions to a high ammonia level in your tank. In the event of elevated ammonia levels, water changes are necessary, as may be the addition of ammonia-locking water conditioner, as directed on the bottle. The above information is presented only to be illustrative of the relationship between different parameters. 

Nitrite (NO2)

Nitrite is a chemical seen in the aquarium as the first byproduct of the nitrogen cycle. It is produced when Nitrosomonas bacteria, the first type of bacteria involved in the cycle, consume ammonia, and then produce nitrite as a waste product. Nitrite is then consumed by the Nitrobacter bacteria, the second type of bacteria involved in the nitrogen cycle.


Nitrite is toxic to fish when present in any amount, damaging the kidney, liver, spleen and nervous system, and causing rapid death.

  • Methylene Blue, an aquarium medication, can help to treat fish suffering from nitrite poisoning by improving oxygen transport, and may prevent death.

Like ammonia, described above, nitrite exists in two forms in the aquarium:

  • NO2 - Nitrite (less toxic)
  • HNO2 - Nitrous acid (more toxic)

and the balance of each form present in the water shifts as pH changes, but in the opposite way as ammonia:

  • In a low (more acidic) pH, more of the HNO2 (toxic) molecules are present;
  • In a high (more basic) pH, more of the NO2 (less toxic) molecules are present.

Nitrate (NO3)

Nitrate (nitric acid) is a chemical seen in the aquarium as the second, and last, byproduct of the nitrogen cycle. It is produced by the Nitrobacter bacteria, the second, and last, type of bacteria involved in the cycle.


Nitrates are far less toxic than either ammonia or nitrite, as the accumulate to high levels in the aquarium, they can and will cause stress, and even death in fish, especially those that are young or sensitive. Its levels should be controlled through regular water changes - with 25% weekly usually being sufficient to keep levels within a the safe range of 0-20PPM.