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Basic Aquarium Water Parameters

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Here's an introductory description of the most important water parameters for the aquarium hobby.

  1. pH (Power of Hydrogen)
    pH is the measurement of hydrogen ions. Increased hydrogen ions (less bonding) result in a drop of the pH (more acidic water), while a decrease results in a pH rise. pH is measured on a scale from 0-14. The neutral value is 7, while values below are more acidic (towards a car battery) and values above 7 more basic (towards dish soap). Changes in pH are a common cause of fish fatalities. Fish can adapt to most pH levels, if not broadly out of range, but they don't adapt well to bouncing values. This is because pH has a logarithmic function (mathematical - meaning ten-fold). In other words, a change in pH from 7 to 6 means 10 times more acidic water. A further drop to a pH of 5 equals 100 times more acidic water. If you have to adjust the pH in your tank, always consider the carbonate hardness (below). The pH in harder water is more difficult to adjust because it bounces back, while softer water is more easily adjusted. Keep in mind to change it slowly as it causes a lot of stress to your fish. Maintaining a stable pH is generally more the way to go.

    Some other facts about pH:

    • Ammonia increases in toxicity with rising pH
    • Nitrifying bacteria experience reduction of growth and action below a pH value of 6

  2. Water Hardness
    • General Hardness (GH)
      GH primarily measures calcium and magnesium ions. It is important for breeders (some species require very soft water, which is hard to maintain, requiring constant monitoring for maximum success). Other then choosing the right fish for the existing conditions, the GH is generally not all that important for the average hobbyist.

      ppm degrees defined as
      0 – 50 0 – 3 soft
      50 - 100 3 – 6 moderate soft
      100 -  200 6 – 12 slight hard
      200 – 300 12 – 18 moderate hard
      300 – 450 18 – 25 hard
      450+ 25+ extreme hard

      Common measurements use ppm and the equivalent mg/l. The conversion of degrees into ppm and mg/l is by multiplication of 17.8 or vice versa.

    • Carbonate Hardness (KH)
      KH measures dissolved bicarbonate and carbonate ions. They are commonly referred to as the buffering capacity. KH determines how stable your pH will be and is therefore very important. Picture this: carbonate ions bond with hydrogen ions (which is your pH). The more bonding the higher the pH. Lesser carbonate ions results in a drop of pH. KH of 70 ppm and less will initiate the pH crash. Baking soda can be used to increase KH and distilled water to decrease KH (as distilled water has a KH of 0). We do not encourage attempting adjustments of these values unless it is absolutely necessary. Should you feel the need to make adjustments, please be cautious and take it slow. Be sure to carefully monitor any changes in KH and pH. If your fish and tank are thriving, it is not recommended that any adjustments be attempted.

  3. Nitrogen Compounds
    Ammonia, Nitrite, Nitrate are well described in The Nitrogen Cycle article.

  4. Phosphate
    Phosphates enter the aquarium mostly through water changes using tap water, but also through food and leaching carbon. Next to chlorine some communities also add phosphates to the tap water. Dying plants and algae will create phosphates while decaying (mineralization). Very interesting to know is the fact that the high pH level required for salt water will hold phosphates in an insoluble stage. A drop in pH, and this happens in a matter of days, will make those compounds water soluble and therefore available to the algae spores. Note that the pH levels usually vary slightly within the tank, and a drop in one spot can have an effect.

  5. Silicate
    The most common points of entry are the substrate, salt, water and dying diatom algae. Keep in mind that R/O and DI units (filters to purify tap water by membranes or by chemical/mechanical processes creating distilled water) will prevent silicates only for a few days, before they find the way through the membranes. Another form of silicates is silicid acid, created by decaying organic matter. Same as phosphates, they can be water insoluble at a high pH, and become readily available with decreasing pH levels.

  6. Chlorine/Chloramine
    Water companies add chlorine or chloramine as a disinfectant to tap water. Chlorine is less stable then chloramine and airs out in just a few days. Some hobbyists simply let the water age for a couple of days before doing the water change, thus airing out the chlorine. Chloramine is much more stable. That is why communities switch from chlorine. As it is very stable, it does not air out even if heavily aerated. Chloramine, a mixture of ammonia and chlorine, passes (unlike chlorine) through the fish’s tissue directly into the bloodstream. In the blood, just like nitrite, it destroys the oxygen carrying cells. Chloramine can cause all fish to die within 24 hours.

    Carbonate hardness, pH, nitrate, and ammonia/nitrite (salinity for marine tanks), should ideally be tested on a weekly basis. Also be sure to test the water used during water changes.

    The water parameters and definitions in this issue are intended to help you understand the conditions in your aquarium. A healthy aquarium requires immediate attention should one of these parameters produce dangers readings.

    Other parameters such as trace elements i.e. iron, copper, calcium etc. should be checked on, if you add them in some way (as supplement or fertilizer).

    Depending on the set up (marine/reef/plants), other factors come in to play, such as dissolved oxygen, redox potential or CO2. These topics will be covered in future issues regarding specific tank types.

Scott Charles
Read other fine articles by this author at
Algone Corporation´s Website
Reader Comments Comentário

First of all, I am a great fan of your website. Therefore this 'tiny' little improvement (or perfection): pH meaning 'power of hydrogen' is only a general assumption (due to a translation from the German word 'potenz'). Which is also a misconception. The H of course stands for the H+ ion, but the little p was merely a convention used by its inventor Sorenson, so it doesn't really have a meaning. My proposal is to call pH 'acidity or alkalinity' (or equal correct explanation) for a more scientific correct approach. Reference: for completeness.

Contributed by Jo Heymans

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