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Water Parameter
Unit(s): none
Target: 8.1 - 8.4
Danger: < 7.8

> 8.6

Tendancy: Decrease
Increased: Photosynthesis
Decreased: Respiration
Testing: Daily
Maintenence: none


The pH of water indicates how acidic or basic the water is. A pH below 7.0 means the water is acidic. A pH above 7.0 means it is basic. The term alkaline is sometimes used interchangeably with basic, but do not get this mixed up with alkalinity, which means something else.

Desired Values in a Marine Aquarium

In a marine aquarium, the pH should ideally be between 8.1 and 8.4, but a range from 7.8 to 8.6 can be tolerated by most organisms. The pH of the water is mainly influenced by the alkalinity of the water and if the alkalinity is 2.5 meq/L or above, the pH is usually within the ideal range.

How to Adjust pH

If the pH is not within the ideal range, it is important to first determine the alkalinity of the water and correct this if required. If the alkalinity is satisfactory, the cause of the non-ideal pH should be determined and corrected. It is unwise to try to adjust a non-ideal pH directly with chemicals as this may just mask a problem. See the links below for causes and cures of pH problems.



My pH is low, how do I fix it?

One of the most common causes of low pH is low alkalinity. pH and Alkaliniy are interrelated and if the alkalinity is low, so will the pH. If everything is right with the water, an alkalinity of 2.5 meq/L should result in a pH between 8.2 and 8.3. If alkalinity is below 2.5 meq/L, you should address this first. See: If alkalinity is low, how can it be raised?

Another common cause of low pH is excessive carbon dioxide. When carbon dioxide dissolves in water it creates an acid which can lower the pH. If alkalinity is 2.5 meq/L or higher, but there is excess carbon dioxide, the pH could be lower than 8.0. The best way to test for excess carbon dioxide is to take a reading of a sample of water taken directly from the tank and note the pH. Take another sample but aerate it (with a pump and airline) outside for 15 minutes and measure the pH. If the pH has increased, there is excess carbon dioxide in the water that is dragging the pH down.

Excess carbon dioxide may be present in the water due to insufficient gas exchange and circulation. This will be most noticeable at night after the lights are out and there is no photosynthesis occurring. In fact, if there is a large difference in the pH of the water between night and day, insufficient gas exchange/circulation is the most likely cause.

Excess carbon dioxide in the water can also result from excess carbon dioxide in the room where the tank is situated. If the room or house is closed and had little ventiliation, carbon dioxide can build up in the air. This is more likely to happen in Winter when the doors and windows are closed and particularly if heating is from combustion (wood, gas, etc.). Increasing ventilation is required if this is the cause.

Should my refugium lighting be opposite to my main tanks lights?

It can be, the reason for this is to help off-set the changes in pH that occur in the main tank during the day/night cycle....

When the Coral and Algae in the main tank are exposed to light, oxygen is produced as a byproduct of the photosynthesis process and carbon dioxide is consumed, this removal of carbon dioxide from the water causes the pH to rise. So when coral and algae are not exposed to light the production of oxygen is reduced and the production of carbon dioxide is increased, which leads to a decrease of pH during the periods of darkness. These fluctuations can be eliminated to a great extent by lighting a refugium in an opposite cycle to the display tank, minimising the pH fluctuations and maintaining the amount of dissolved oxygen constant in the water.