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Home > Articles > Index.php?cont_id=4

id:4 Title:the pH scale Date:27-01-2011
Short Detail:litmus test - acid or alkaline, the pH scale


litmus test - acid or alkaline, the pH scale 


What is pH?


The pH scale is a scale of acidity & alkalinity. The scale runs from 0 (most acid – such as battery acid) to 14 (most alkaline – such as drain cleaner). A neutral solution has a mid-range pH level of 7. The scale is logarithmic so for example a glass of beer with a pH level of 4 is ten times more acidic than a cup of black coffee with a pH level of 5.


The pH (short for ‘power of hydrogen’) measures the concentration of hydrogen ions, H-, in water. Therefore the pH scale can only be used for solutions of acids and bases in water. A solution is said to be acidic when the pH level is less than the mid-range pH of 7, and basic (alkaline) when the pH level is greater than 7.


Contact with extreme alkaline water (with a pH level above 12) can cause blistering or burning of the skin and lead to digestive problems if drunk. Some common pH values are shown below:


pH scale



Measuring pH


The pH value or level can be easily measured using a home test kit, readily available at any aquarist supplier. The kits range from the simple inexpensive strips to complex vials and cylinders.



Ideal pH


The pH value of the water is important as it influences the functions way of your fish’s body. The pH range of interest to the tropical fish keepers is between 5 & 9, as most freshwater fish prefer slightly acidic water to neutral water in a range from about 6 – 8. 


Different fish do require different pH levels. For example, the ideal pH level for Oscar fish is between 6.8 and 7.2, but for many types of Cichlids around 8 is recommended. However many fish have been bred for many generations in the aquarium and have already become accustomed to water conditions quite different to their natural habitat. As with temperature, aquarium fish tolerate a wide range of pH levels but most of them prefer a much narrower range. Research may help you avoid tragedy as some species do require a particular range of acidity or alkalinity and they will not thrive outside of that range.



Maintaining a steady pH


Fish respiration, excrement and other sediments together with the effect of plants dictate a tanks pH level. Under normal conditions, the pH level in your aquarium will vary a little over time. If the pH level drops below 5, bacteria levels decrease to a point that wastes cannot be broken down and most fish will not survive.


The pH range quoted for a given species may be based on its native waters. Although it might be desirable to mimic these conditions to some extent, the fish may be quite capable of thriving at a slightly different pH level. Providing a stable pH level can be more important than the exact value, as long as extremes are avoided.


Averaging out the suggested pH levels may be an effective means of establishing which pH level is best for your community tank. However, the most important thing about the pH level of your tropical fish tank is that you should keep it steady as short-term fluctuations in the pH level will upset the body chemistry of the fish and may lead to health problems. 


Keeping the pH level stable is especially important when changing your tank water, which should be done regularly to ensure a clean living environment for the fish. Adding make-up water and performing partial water changes alter the pH level to some extent. Hard water is typically a little alkaline due to carbon dioxide reacting with carbonates and bicarbonates in the water. 


Before changing the water, measure the current pH level in the tank and then adjust the make-up water to match that level. This adjustment must be done before putting the fish into the make-up water, or you risk shocking their systems. Performing frequent partial water changes, in addition to proper cleaning and good aeration help keep the water pH level stable. It is useful to know the pH level of your tap water (or other source), to determine its suitability for the fish you wish to keep. Measure the pH level after the water has been left in a bucket overnight, as the pH level may change due to gases dissolving into, or being released from, the water.



Changing the pH 


Before attempting to change the pH level of your aquarium water, you should ask yourself if it is really necessary to do so. Often, good aeration and filtration, together with good housekeeping will eliminate the need for any chemical adjustment to the pH level of your tank. Decaying plants for example can lower the pH level. Good housekeeping helps maintain a more stable pH level. 


There are of course some fish which do require specific conditions to thrive. You may also want to alter the water chemistry to improve success with breeding, or to improve the growth of demanding plants.


Increasing the pH level is usually easier than lowering it, and will usually involve raising hardness at the same time, in order to keep the pH level stable. It can be achieved through the use of decor containing buffering salts, such as limestone rock, the use of crushed coral in the filter or commercial buffers and “pH-up” products.


Lowering the pH level can be more difficult, particularly in hard water which has a good buffering capacity. It can be achieved through filtration through peat (this will be more effective in water with a lower carbonate hardness (KH)), commercial “pH-down” products. Again, these will not work effectively where there is a strong buffering capacity. Some will also introduce phosphate to the aquarium which will encourage undesirable algae growth.


Attempting to lower the pH level of well-buffered water with commercial chemicals or acid solutions is likely to result in a losing battle, as the buffer system causes the pH level to re-stabilise at its original value. The resultant swings of the pH level are however likely to be harmful to your fish. The solution is to reduce the buffering capacity or KH first.


Apart from the effect of the pH level itself, there are important effects on the toxicity of ammonia and nitrite with changing the pH level. Therefore you should be particularly wary of attempting to change the pH level when either of these waste products is detectable in particular, during the cycle. It is safer to let the cycle finish before attempting to adjust the pH level as it may settle at a different value once the cycle is complete in any case.



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