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What Is pH And How Does It Affect Your Fish?

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pH defines how acidic or basic the water is. The term "pH" describes the amount of hydrogen (H+) and hydroxide (OH-) ions dissolved in a solution. (For those who are interested, in mathematical terms pH is defined at the negative log of the hydrogen ion concentration.) 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 a 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.

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 under-gravel filter. Excess waste product produces carbonic acid, which acidifies the water and lowers the pH.

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

Filtering water over peat

Add bogwood to the tank

Inject carbon dioxide CO2

Use a commercial acid buffer

Water changes with softened water or RO/DI (Reverse Osmosis/De-Ionized) water

Ways to raise the pH

Aerate the water, to drive off excess 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


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