What Is ORP in a Reef Tank? (Oxidation Reduction Potential)

Life in an aquatic ecosystem depends not only on its active reaction (pH index) but also on the oxidation-reduction potential (ORP) or redox potential. The redox potential, or redox concentration, is an indicator of water’s chemical and biological activity. 

What is it, why is it important and what is it for? Read this article to find out the answers to all these questions.

What is the ORP?

Oxidation-Reduction Potential (ORP) is the ability of a substance to reduce or oxidize another substance. RedOx potential stimulates or inhibits the growth and development of aquatic organisms.

The word redox is formed from two words  — reduction (reduction) and oxidation (oxidation). The reduction would be the process of releasing oxygen or absorbing hydrogen; oxidation would be the process of absorbing oxygen. During oxidation or reduction reactions, the electrical potential of the substance being oxidized or reduced changes  — one substance, giving up its electrons and being positively charged, is oxidized; another, gaining electrons and being negatively charged, is reduced. The electric potential difference between them is the redox potential. 

In measurements, the value of this difference is designated as Eh and expressed in millivolts. In aquarium water Eh ranges from – 400 to + 700 mV, is determined by the number of oxidation and reduction processes.

Redox potentials are measured in millivolts and, by analogy with pH, can be recalculated either in gH, reflecting hydrogen pressure, or in Еh, the index of electron activity. Oxygen has the best oxidizing capacity, while hydrogen is linked with the best reductive abilities. However, there are other substances in between. Some of them act like oxidizers, others  — reducing agents. 

Oxygen and ozone are of particular interest for reef keepers. Oxygen is opposed by a complex consisting mainly of nitrogen-containing substances formed during the decomposition of organic residues and a group of metal compounds with variable valence (e.g., iron). Ozone is the next strongest oxidizer.

Within the water column, the atmosphere plays the role of an oxidant supplier and the bottom plays the role of a reducing agent supplier. Ultraviolet irradiators and ozonators strongly increase the redox potential. 

Why you should know the value of the rH?

The reason why aquarists pay attention to the redox potential is that certain plant species behave differently in aquarium water, their reproduction and development is either stimulated or, conversely, inhibited. Aquatic plants in running water require higher values – rH, they are usually more difficult to maintain. Plants growing in still water (ponds and marshes) require a lower value of redox potential.

The value of redox potential in the aquarium should be observed because it allows you to clearly understand whether microelements are available to be digested or not. If the rH value is high, most of the substances are in the form of compounds with oxygen, so plants can not absorb them. 

Additionally, rH shows water saturation degree or simply put  — the pollution rate within the display tank. Most of the life products (fish)  are reducing agents. So, the accumulation of fish poops causes decreased rH, especially closer to the sandbed.

Oxidation indicates the amount of organic matter in the water (primarily fish excrement and uneaten food, as well as the products of their decomposition). Dissolved oxygen in the water is consumed not only for breathing fish but also for the oxidation of organic and some inorganic substances in the aquarium water.

Knowing how to manage the value of redox potential is especially important when using ozone in the aquarium. In other cases, parameters such as pH, temperature, alkalinity, calcium content, availability and accessibility of suitable food, and many more have a much more important and noticeable effect on the development and growth of invertebrates in reef aquariums.

What are the units of measurement for the redox potential?

There are special tables to convert the results measured with the device in millivolts to rH units. The scale of conditional units contains 42 divisions, rH = 0 means pure hydrogen  — corresponds to a system with very strong reducing properties, while rH = 42  — pure oxygen  — corresponds to the potential of the so-called oxygen electrode. 

An increase in rH corresponds to an increase in the oxidative properties of the system. Usually, a system with rH above 25 is referred to as an oxidizing system, and with rH less than 20  — as a reducing system. It should not be forgotten, however, that the concepts of oxidative and reductive are relative. Just as only part of the pH range is applicable in reef keeping, only part of the rH range is acceptable to fish and plant life. This is the range from 16 to 36.

rH 40-42 Atmospheric oxygen (most oxidizing)
rH 35Strong oxidation
rH 30Moderate oxidation
rH 25Weak oxidation
rH 20Weak reduction
rH 15Moderate reduction
rH 10Strong reduction
rH 0.5Atmospheric hydrogen (highest reduction)

Table.1. Different rH levels

Anaerobic (living in the absence of free molecular oxygen) organisms prefer low rH. For example,  bacteria, fungi, diatoms. These conditions are formed in aquariums with sharp water substitution and the accumulation of decomposed feces, rotten plant pieces, dead snails, etc. 

Low rH also occurs within the thickness of the caked fine-grained soil, deprived of access to water, saturated with oxygen. Soil is the “kitchen” that determines the total index of redox potential in the aquarium – the more substance accumulation in the soil (tend to give off electrons), the more rH decreases. 

If you care about the health of your tank, you better keep it clean & tidy. Clean the substrate, watch the sump, remove any wastes as soon as possible.  Elementary cleaning once or twice a week ensures your animals are safe and sound. 

How to measure the rH?

The rH of water is measured electrochemically, using glass electrodes. The natural ORP level of water is between -400 and +700 mV and is highly dependent on the ambient temperature.

How does the rH measuring work?

The easiest way to get objective information is to contact one of the specialized laboratories that study water. But when information is needed regularly, it is worth thinking about buying your own ORP meter.

If you are interested in a device that can make accurate measurements, it is worth paying attention to the models that have long proven themselves in this area. These are usually SanXin, Kelilong, and Mettler.

Indicative rH values for some aquarium plants

  • rH 28,3-28,5 – The aquarium requires regular water changes and siphoning of the sandbed. Cryptocoryne, marshweeds, burheads, and other marsh plants grow and multiply well; cryptocoryne plants tolerate slightly lower.
  • rH 25,6. Plants such as hornwort, najas grow well in a wider range (30-26) and are the most unpretentious plant species.
  • pH 29-30 – Aquarium gravel is silted up a bit, you can make the first (small) siphoning of the soil. Most aquarium plants develop normally, grow well many long-stemmed plants, Ludwigia, Hygrophiles (a.k.a phyton), Waterweeds, red-leaved species grow bad, etc. 
  • gH 30,2-30,6 – Wallisneria, Rotas, Burhead, and Aponogetons bloom abundantly; Cryptocoryne loses their leaves in such an environment.
  • pH 30,7-31 – This is a newly arranged aquarium, the soil is still weakly silted. Well developed, Cabomba, Synnema, aponogetons discard leaves, Cryptocoryne rotting roots, Burhead stops growing.
  • pH 30-32 – Favorable conditions for the development of green algae in the aquarium. In this case, during the outbreak of floating algae “bloom water” – water substitution with fresh causes their multiplication.

How to increase the rH in an aquarium?

There are several ways to increase the rH in the aquarium:

  • Raise the rH value can be through water changes, aeration, and removal of waste products of the aquarium inhabitants, cleaning the ground. Oxidation products are removed by water changes, filtration through mechanical filters and activated carbon, and the installation of a pepo-separator. 
  • One of the most acceptable options is the use of ozone. It should be said that high-quality water does not require the addition of ozone to keep the redox potential at this level
  • Theoretically, a very strong oxidizing agent is potassium permanganate – manganese oxide (KMnO4). But it should be used cautiously and only as a last resort when all other methods have been used. Permanent use of potassium permanganate in the aquarium can not
  • To increase the redox potential is also sometimes used hydrogen peroxide, better known as hydrogen peroxide, however, should say about the inconsistent results of hydrogen peroxide in aquariums. The failures appear to have been due to too rapid a rise in the redox potential in the aquarium.

What factors reduce the level of rH?

Here is the list of common factors that causes the redox potential drop: 

  • Overfeeding the animals or feeding them too often 
  • Too much decaying material of plant and animal origin in the tank (dead fish, dying invertebrates, decaying plants) 
  • Filters clogged with dirt, especially mechanical filters 
  • Dirty bottom ground 
  • Activated carbon that was not changed in time 
  • Overcrowded aquarium
  • Unsuitable biofilter filler
  • Presence of nitrite in the water
  • Low dissolved oxygen level
  • Too many different organic additives, trace elements, liquid fertilizers

Bear in mind that other reasons may contribute as well. If you’re having a problem with redox potential, take the list and check step by step.

How rH related to pH?

pH and rH values are closely related. Oxidative processes decrease the active reaction of water (the higher the rH, the lower the pH), reductive processes increase the pH. In turn, the pH indicator affects the rH value. Thus, the rapid process of photosynthesis changes the value of rH in thickets of plants such as elodea and Cabomba, capable of extracting CO2 from bicarbonates during photosynthesis: as a result of emitted ÎÍ- ion, alkalizing water, and the index rH decreases, while in other areas of the aquarium, it may remain unchanged. 

It should also be noted that the rH value in the upper layers of water is usually higher, while the lower ones are lower. Since pH values fluctuate throughout the day, the value of rH changes. Another influencing factor is water temperature.  Thus, the aquatic environment is constantly undergoing both oxidation and reduction reactions that are not visible to the aquarist’s eye. 

Bart Sprenkels

I have been keeping multiple aquariums since I was 18 years old. Just like many of you, I started with two goldfish but quickly learned they were not suitable for aquariums. Later, I switched to a tropical community tank and I also have two pet musk-turtles in a bigger aquarium. You can read more about me here.

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