Ph In The Marine Aquarium

The quality of the seawater in the aquarium is of paramount importance. There are basic parameters that need to be correct and stable, and one of these is pH.

There isn’t going to be any scientific mumbo jumbo here, but the foundations of pH need to be understood. pH is a measurement of the acidity or alkalinity of the seawater. On the pH scale, 7 is neutral, 0 is acid, and 14 is alkaline. (Just for interest, sulphuric acid measures 0, and sodium hydroxide measures 14.)

The pH area that the marine aquarist is interested in is from pH 8.0 to 8.4, so it can be seen that our seawater is on the alkaline side. The often quoted ‘perfect’ pH measurement is 8.3, and aquarists do achieve this. However, if the seawater is not 8.3 there isn’t a need for concern, for successful marine systems can run at the stated 8.0 to 8.4. In fact, a few successful systems run at 7.9. The important part is stability. The pH shouldn’t see-saw greatly as this is detrimental to the livestock.

The life functions of the livestock creates pressure on the stability of the pH. This pressure is pushing the pH towards the acid side of the scale. In the confines of the aquarium this needs to be monitored and controlled.

To find out the pH of the seawater is simple, and requires the purchase of a pH test kit suitable for seawater. These are easily obtainable and should be one of the test kits in use weekly at least.

There is another reason why the pH can fall, and this is because of the day/night cycle. During the day the pH is at the highest point, and at night, when the lights have been off for a few hours, it is at the lowest. This is not always the case – in my aquarium the pH is a constant 8.0, day and night.

Any fall is easily discovered, just do a test after the lights have been on a few hours and after they have been off a few hours. If the fall is slight, there isn’t a problem, but if it is large then action can be taken. Many aquarists use a sump and plant it with the macro algae Caulerpa. The Caulerpa is lit when the main aquarium lights are off and this helps counteract the pH fall.

As has been said, the pH is under pressure to move towards the acidic side of the scale. There is resistance to this pressure by what is termed the buffering capacity of the seawater, or the alkalinity. This alkalinity is mainly because of the carbonate and bicarbonate content of the seawater.

The aquarist should be completing routine seawater changes, as this helps to maintain the alkalinity and thus helps stabilise the pH. However, some aquarists find that they need to protect the pH from falling by additional methods. This is simply done by using a commercially available dry powder, mainly carbonate and bicarbonate, and often called ‘pH Buffer’ or ’Alkalinity Booster’ or similar. The instructions provided should be adhered to. There are other ways which will not be covered here.


Ph In The Marine Aquarium
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6 Comments
  1. For me this is one of the most overlooked parameter, I tend to lack on testing it as often as I should…but for new aquarist they should focus on it as it is the root of problems alot of the time….Great article:)

  2. “Many aquarists use a sump and plant it with the macro algae Caulerpa. The Caulerpa is lit when the main aquarium lights are off and this helps counteract the pH fall.”

    Getting more technical, the reason why adding a macro algae like Caulerpa or Cheato increases the pH is because of the increased Oxygen levels in the aquariums.

    For the beginners remember that Carbon Dioxide (CO2) decreases pH and Oxygen (O2) increases pH. As a result, covered aquariums in basements may have a lower natural pH due to lower gas exchange and ventalation.

  3. but how do you correct ph if it went low or high?

  4. What’s the best way to increase my purple algae growth?

  5. Hello. You’re referring to the ‘solid’ encrusting algae called “coraline”. It’s assumed you run a reef aquarium. First, the lighting should be up to spec, that is the bulbs/tubes are not old and ready for renewal. Also seawater quality should be high with low phosphate (preferably testing ‘undetected’ or not more than 0.03) and nitrate not more than hopefully 10ppm.
    When cleaning coraline from glass etc this action itself helps to spread it. It has also been found (as in my own aquarium) that a higher alkalinity helps. If the alkalinity is raised to 4.0meq/l and maintained at this – deviations over say a week don’t matter – then coraline and perhaps other potential decorative algaes are encouraged. Some suggest a level of 3.2 meq/l (which in dKH is 9). Of course alkalinity supports the presence of good algae, it doesn’t introduce it. Test kits are readily available as are alkalinity supplements sometimes called KH buffers.

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