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pH is arguably one of the most important environmental parameters for any biological system. It is a measure of hydrogen ion concentration and determines whether a solution is an acid or a base. Most biological systems tolerate only a very limited pH range. Stuff this one up and your critters start dying... quickly.

Target Value:
8.4 (8.0 - 8.6)

More Detail:
pH is a parameter that measures the degree to which a solution is acidic or basic on a scale from 1 to 14. Pure water has a pH of 7. Acids cause a pH that is less than 7, with smaller values representing stronger acids. Bases cause a pH that is greater than 7 (the bigger the value, the stronger the base). Most water in natural environments usually has a pH between 5 and 9.

pH is measured on a logarithmic scale, meaning that water with a pH of 5 is ten times more acidic than a solution with a pH of 6. A small change in pH reflects a large change in the quality of the water.

Most people are aware that H20 is another name for water, but many are unfamiliar with what this means. Water is a molecule made up of two hydrogen atoms, connected to an oxygen atom.

In a glass of water (or an aquarium for that mater), most of the water molecules are in the form shown in the diagram to the left, however, a small percentage of the water molecules will break up into two parts:
one part made up of a single hydrogen, the other part made up the oxygen plus the remaining hydrogen. The single hydrogen is referred to as an hydrogen ion, while the oxygen-hydrogen part is called an hydroxide ion.

Chemists would say that the water molecule has dissociated and would write the above using the equation:

H20 --> H+ + OH-

How many molecules does this happen to? If you were to count up all the normal molecules and all the dissociated molecules in a glass of water (not recommended), you would find that only one molecule in every ten million would have dissociated. This gives a concentration of hydrogen ions that can be written as [H+] = 10-7. The square braces are shorthand for "concentration of". Because there is one hydroxide ion for every hydrogen ion in pure water, the concentration of hydroxide ions is also one in ten million: [OH-] = 10-7.

O.K., what's the point?

Free hydrogen ions are chemically reactive i.e. they "eat" things. By adding certain chemicals to water, we can increase the concentration of hydrogen ions, making the water more reactive - we have created an acid. Similarly, hydroxide ions are also reactive. By increasing hydroxide ion concentration, we create a base.

Both acids and bases are capable of corroding many materials. Hydrochloric acid is found in our stomaches, and helps us to digest food. Ever had the burning sensation in the back of your throat after vomiting? That's acid. The stuff used to clear clogged drains is a base - caustic soda or sodium hydroxide (whichever you prefer).

Obviously, the creatures in out care would not fare to well in either hydrochloric acid or caustic soda, so we need to measure and regulate the pH in our aquariums.

Formally, pH can be expressed as:

pH = -log10([H+])

Note: As [H+] increases, [OH-] decreases such that [H+]x[OH-] = 10-14 for all water-based solutions.

What value should the pH be in my aquarium?
pH in a marine aquarium should be between 8.2 and 8.4. A little lower or a little higher is O.K. (8.0 to 8.6). Outside of this range, you are probably asking for trouble. Like many parameters in a reef aquarium, stability is more important than absolute value. If you need to change the pH, go slowly.

Note: Delbeek and Sprung suggest that the optimum pH for calcification of stony corals is around 8.4

Why is pH important?
pH is an important biological parameter for all organisms. Inside the human body, many mechanisms work hard to maintain our internal pH between 7.35 and 7.45. Values above or below this cause major distress. Diabetics who do not receive sufficient insulin can suffer from a condition called ketoacidosis, where the blood pH can drop to 6.9. This pH can cause fitting, followed by coma and death. All of the chemical reactions taking place in our cells are optimised for a pH between 7.35 and 7.45. We are, however, very good at insulating our internal chemistry from the external environment. We could sit in a bath with a pH of 5 and feel absolutely no ill effects.

In an aquarium, many of the organisms we keep do not have the ability to insulate themselves so well. Shifts in pH cause major stress that can affect fish an corals in one of two ways.

Sudden shifts (more than a couple of points) or sustained values that are way too high or low, will directly and immediately affect the internal biology of an organism and may kill it outright.

Long term maintenance of pH outside of the target range, makes the organism work harder than it should to maintain its internal chemistry, causing stress, which weakens the immune system and makes organisms subject to disease.

Also, the chemical reactions that allow calcification in corals will only occur over a limited pH range.

How do I measure pH?
pH is measured with either a pH test kit or an electronic pH meter. Measuring pH with a test kit usually involves taking a sample of aquarium water in a tube or vessel, and adding a dye that changes colour according to the pH (called an indicator). The resultant water colour is compared to a chart, and the pH is estimated.

Using a pH meter simply involves sticking the thing in a sample of aquarium water and reading the value of a digital display.

What influences pH in my aquarium?
Aquarium pH can change due to the following causes:

The bacteria on live rock and in live sand that convert ammonia to nitrite and nitrite to nitrate tend to add H+ molecules to the water, causing it to become more acidic.

All organisms need to breath. Oxygen is taken up and used to burn food which in turn supplies the energy that sustains life. The by-product of this process is carbon dioxide (CO2). Carbon dioxide dissolves in water then reacts to form a weak acid, which lowers pH.

CO2 + H2O --> 2H+ + CO32-

During the day, plant life in the aquarium (including zooxanthalae in corals) use more carbon dioxide than it produces. The CO2 is used in a process called photosynthesis. This reduces the amount of CO2 in the water, increasing the pH. As a result, most aquariums cycle through a pH range during the day. First thing in the morning, just before the light come up, the pH will beat its lowest. Last thing at night, the pH will be at it's highest value.

Calcium reactors:
Used incorrectly, calcium reactors can lower the pH of an aquarium by introducing excess CO2.

How do I manage the pH of my aquarium?
The typical cycle of falling pH over night and rising pH during the day can cause the pH to wander outside of the ideal range. Improved gas exchange can help dissipate excess carbon dioxide from the water. This is best achieved using a good protein skimmer and/or power heads directed at the surface of the water.

Maintaining the alkalinity at the desired level is also essential for pH stability.

How do I alter the pH in my aquarium?
The pH of an aquarium can be altered by adding appropriate chemicals in correct dosages. Alterations in both directions may be achieved.

There are a couple of ways to increase the pH of a reef aquarium. The first method involves the addition of a product called a marine buffer. Buffers do not directly affect the pH, but instead act by increasing alkalinity thus stabilizing (and often increasing) pH. The interaction between pH and alkalinity is quite complex, and is described in the alkalinity section. Any resulting increase in pH takes time (a couple of days or so).

I have personally used Kent, but any high quality product will suffice.

The second method involves adding a chemical that is naturally a base. The chemicals Calcium Oxide or Calcium Hydroxide both have this property (referred to by the German expression kalkwasser when dissolved in water). Both are used to increase calcium ion concentration, but both also have the net result of directly and immediately raising the pH of the aquarium water.

CaO + H2O --> Ca2+ + 2OH-
Ca(OH)2 --> Ca2+ + 2OH-

Addition of Kalkwasser should be performed slowly and preferably over night, when the pH of the aquarium is inclined to drop naturally.

Adding too much Kalkwasser at once can raise the pH to unsafe levels, stressing the animals in the aquarium, as well as potentially causing some calcium to fall out of solution, reducing calcium levels (exactly the opposite affect usually intended).

In my experience, the need to lower pH seems to be less often an issue than raising pH. Some systems, however, seem inclined toward high pH. The mildest way to decrease pH is CO2 injection. This involves the use of a carbon dioxide cylinder, a pH controller and a gas solenoid to control the flow of gas... a somewhat expensive option.

What problems may I experience if the pH in my aquarium is outside of the target range?
Many problems can occur if the pH is maintained outside of the target range. A group of common problems is given below.

Too low:
  • Excessive algae growth
  • Poor coral growth
  • Poor coral health
  • Pulsing Xenia cease pulsing
  • Distressed fish
  • Organism death
Too high:
  • Poor coral growth
  • Poor coral health
  • Difficulty maintaining calcium levels
  • Difficulty maintaining alkalinity
  • Clumping of the substrate
  • Distressed fish
  • Organism death