The pH value is an important value in reefkeeping which often is neglected. The optimum pH value for a reef tank is between 8.1 and 8.4, and preferably as stable as possible. Unfortunately, our tanks rarely show this pH stability. Photosynthesis of corals and bacterial activity have significant influence on the acid content of the limited amount of water. In the course of a few months, the pH usually decreases and mostly settles between 7.6 and 8.1. The aquarium lighting also causes day/night fluctuations which can build up to 0.5 pH units. Consequently within 24 hours the acidity of the tank water might triple or decrease accordingly. These fluctuations have a great influence on biofilms and the nutrient concentration in the tank because at low pH values more PO₄³⁻ is released from phosphate deposits. For corals, such fluctuations represent stress and prevent them from developing optimum coloration and growth.

What’s that:

The pH value indicates whether a water is acidic or basic. Seawater has a slightly alkaline character and should have a pH value of 8.0–8.3 in the aquarium.


A too low or too high pH affects many biological processes in the aquarium. For example, deposits of phosphates and trace elements are released from the rock decoration surface if the pH value is too low, the opposite effect develops if the pH value is too high. This leads to strong nutrient fluctuations, algae growth, cyanobacteria and dinoflagellate deposits. A stable pH value therefore is important. During 24 hours of the day, a pH fluctuation up to 0.3 is considered normal.

Indicator species:

The first signs are a sudden increase in nutrients and fluctuating PO₄³⁻ levels, thin or missing growth tips on hard corals, and dying coralline algae. Soft corals like Xenia contract their polyps.

Value too high:

Check skimmer, aerate tank, change carbonate addition, reduce illumination times.

Value too low:

Check skimmer, install Skim Breeze CO2 adsorber, improve ventilation. Improve flow, remove possible rotten spots. Reduce nutrients and carbon additions. Check boron and zinc values. If lime reactors are used, add an outlet filter with broken lime to bind CO2.

Variety Water value
Benefit Acid stability of the water
Reference value 8,3
Skill Level green
Source Salt mix, supplements
Available Balling Light Set, Skim Breeze, KH Mix, Balling Salt
Importance 1–6 6
Detection quality safe
Relation values Alkalinity
Fluctuation margin preferably no fluctuation

We recommend regular pH tests and appropriate measuring to avoid strong pH fluctuations:

  • Control of the skimmer
  • ensure adequate ventilation of the room
  • Install Skim Breeze CO2 filter media
  • Improve flow
  • Reduce nutrients
  • Keep an eye on boron and zinc values, if necessary supplement via Elementals Trace
  • Regular replacement of the substrate
  • Change the supply system to Balling Light
  • Reduce the dosage of short-chain carbon sources
  • Dose appropriate bacteria preparations

Balling Light:

With Balling Light, the pH value is kept stable by the supply. The Balling Light system has been designed for continuous aquarium operation. The salt mixtures used in Balling Light stabilize the buffer capacity of the aquarium even with very high stocking densities. Many coral farms use our system for professional coral propagation.

In order to avoiding pH fluctuations, you should take a close look at the material cycles in your aquarium and take measures to prevent fluctuations from the very beginning. In this respect, the acid binding capacity, in simple terms the carbonate hardness, plays an important role. Seawater is a base fluid with alkaline properties, i.e. it buffers acids. This is why it is also called a buffering system. The salts and carbonates contained in seawater buffer the acid and keep the pH value stable. In a closed system like an aquarium, which also has a relatively small volume, some influences have a particularly strong effect on the pH value and constantly stress the buffer system.

  1. CO2-rich room air: Aquaria are mostly located in a closed room. Here you usually find a higher CO2 concentration in the air than outside of buildings, often twice as high. The skimmer transports a large amount of room air into the water. Via gas exchange, the CO2 gets into the water where it forms carbonic acid. This acid decreases the pH value.
    Remedy: Move the air intake of the skimmer to the outside or use a CO2 filter (Skim Breeze).
    Result: pH increase of up to 0.5 units and stabilization of pH fluctuations.
  2. Calcium reactor: By adding CO2 to the reactor, the water is acidified and CO2 enters the water via return flow.
    Remedy: Install a filter column filled with coral sand, so the reactor outflow water has to pass the material before returning to the tank.
    Result: Increasing pH value up to 0.3 units and stabilization of pH fluctuations.
  3. Biofilms and nitrogen cycle: In contrast to natural marine habitats, aquaria have considerably more solid surfaces in relation to water volume. Coral sand, rocks, but also all glass surfaces and technical accessories are overgrown by biofilms.

A stable system with a functional nitrogen cycle is pH-neutral, because the carbon cycle which is coupled with the nitrogen cycle stabilizes the system. In modern aquaristics, however, it has become common practice to promote these bacterial biofilms by adding bacteria or to employ an addition of short-chain carbons to control nutrient concentrations. In this process, parts of the bacterial strains that produce nitrate are inhibited, while those that break down nitrate are promoted. However, the addition of carbons changes the course of the nitrogen cycle, the nitrogen reducing acids are not utilized by the nitrogen building ones. Over time, the pH value in the aquarium therefore becomes unstable and decreases.

Remedy: Avoid dosing short-chain alcohols and sugar. These additives can be used in the short term and also have positive effects, but you should never use them in the long term. This also applies to other preparations containing such alcohols; here too, the dosage should be as low as possible, they are no substitute for good skimming or regular partial water changes. In this context also avoid puffed up salts mixes that advertise any ominous additives. Mostly these are only amino acids, starch, sugar or vitamins, which may give supposedly good results in the beginning, but in the long run they clearly disturb the system.

Why is it that way??

Due to the enormous amount of bacterially active surfaces and the increased bacterial activity we increase the depot effect in the aquarium, because bacteria absorb nutrients and trace substances and store it in biofilms. The one-sided feeding of the bacteria mainly promotes denitrifying bacteria. In addition, a chemical-physical process takes place in which phosphate compounds are deposited on calcium surfaces. In the process, micro- and trace elements as well as nutrients are deposited on the surfaces. Due to increased bacterial activity and nutrient input into the aquarium, large quantities of nutrients are quickly deposited. If the corals grow vigorously, these nutrients are quickly being consumed, but slow growth leads to enrichment.

Calcareous surfaces such as coral sand have an enormously high binding capacity that is comparable to that of adsorber filter material. Regular replacement of the gravel is therefore helpful for nutrient control and stabilisation of the pH-value. More than 15 years ago we developed a method to control the depot effect in the aquarium. See HTU “Heat test according to Schuhmacher”.

Coral growth and photosynthesis

Through photosynthesis, corals and some groups of bacteria release more oxygen to the water during the day and absorb CO2. This increases the pH value of the water towards the evening. During the night this process is reversed and CO2 is released into the water. Here “Skim Breeze” has a particularly stabilizing effect because this CO2 is then released into the outside air via the skimmer. This helps to permanently increase the pH value and thus create more stable conditions in the aquarium. When the corals grow, substances are also released which have a negative effect on the pH value. Therefore, sufficient buffering of the water is important, especially in aquariums with strong coral growth.


The excretions of the fish are the best coral food. Fish exhale nitrogen as ammonium, and corals prefer to use this source because they can directly utilise this form of nitrogen. The feces of fish are also utilised. Good feeding of the fish is therefore very important if you want to achive good coral growth and thus want to limit the nutrient level. Important here are feeds of the new generation (genuine soft granules with a very high content of proteins and fats, provided that these are mainly derived from unsaturated fatty acids). It is also very important that you use feed that is made from whole fish and/or shrimp. Soy or other substitutes are less suitable, as they quickly enrich the tank water with nutrients and inhibit coral growth.

In summary, good nutrient conversion by the corals supports the stability of the nitrogen cycle and thus also the carbon cycle, which keeps the pH stable during nitrogen conversion.