How To Lower ORP In A Reef Tank


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ORP is lowered in reef tanks by adding metals (iron and manganese), most organic molecules (antioxidant vitamins such as vitamin C), inorganic molecules (iodide, sulfide, nitrite, and ammonia), and water conditioners into the water, plus UV sterilization methods. 

Oxidation-reduction potential, or ORP, measures water’s ability to break down waste products like contaminants and decaying matter from plants and animals. When the ORP value is high in reef tanks, it means there is a lot of oxygen in the water.

Many people believe that ORP levels in reef tanks are linked to the “clarity” or “purity” of aquarium water. This common theory is not true, as ORP in reef tanks is a little more complicated, plus the chemicals that control ORP in your aquarium may differ from other reef hobbyist’s aquariums or the ORP levels in natural seawater. Therefore, ORP cannot be the universal way to determine water clarity.

What Is ORP?

Oxidation-reduction potential (ORP), is, without a doubt, the most complicated chemical element in reef tanks that aquarium hobbyists will often encounter. 

ORP is the electron exchange potential that relates to an active oxidant or reductant. Oxidizing agents steal electrons from other molecules, and reducing agents give up electrons to other molecules. 

A simple way to understand ORP is to think of it as a fighting battle against oxidizers and reducers. You want to keep things balanced, not letting one side win the battle. However, ORP values will constantly vary within the range, so you can expect some minor fluctuations. 

So, if you add an oxidizer, it will increase the ORP of the water, and if more reducers are added it will decrease the ORP level. 

Why Is ORP Important In Reef Tanks?

Measuring ORP levels inside your reef tank allows you to evaluate a molecule’s potential to oxidize. ORP is a complicated, yet interesting way to measure water chemistry in aquariums. 

Aquarists that measure ORP are more likely to monitor unusual events that may otherwise be difficult to detect. For example, aquarium hobbyists that experience rapid death of fish or other marine organisms or long-term increases in organic materials can monitor ORP to help maintain their reef tank. This maintenance may include increasing the aeration, skimming, or using carbon dioxide (CO2) scrubbers.

It is also important to note that substances like dissolved oxygen and chlorine will contribute to higher ORP levels. These substances have higher oxidizing agents (electronegativity) and will usually pull electrons from other substances like unwanted contaminants and other bacteria in your reef tank. 

You should avoid ORP levels that are too high, as it can kill all the beneficial bacteria inside your reef tank.

Ideal ORP Levels For Reef Tanks

The ocean has an average ORP level between 300 and 450 millivolts (mV). As it is often difficult to maintain this range in reef tanks, as long as the ORP level is between 200 and 500 mV, your fish and plants will be fine.

What Causes High ORP In Reef Tanks?

A reef tank should have a relatively high ORP. Reef tanks with higher ORP levels are healthy, however, if your levels are greater than 500 mV, you should act fairly fast to decrease them. 

When ORP levels are too high, it means there is too much oxygen in the water from adding extreme oxidizers such as Ozone. Your increased ORP levels could also be from inaccurate readings. Therefore, always ensure your ORP probe does not have any algae tangled around it, and try calibrating your ORP probe

Types Of Oxidizers In Aquariums

  • Oxygen (O2).
  • Ozone (O3).
  • Hydrogen peroxide (H2O2).
  • Hydroxyl radical (OH).

Lowering ORP In Reef Tanks

To lower the ORP in a reef tank, the water, which is dominated by atoms, molecules, or ions, must give away some electrons. Now not all oxidizers and reducers can react with each other, therefore, there is only a subset of oxidizers and reducers that can perform the exchange of electrons.

Reducers In Aquariums

  • Metals (for example, iron; Fe, manganese; Mn).
  • Most organic molecules (for example, vitamin C and other antioxidant vitamins).
  • Inorganic molecules (iodide; I, sulfide; S, nitrite; NO2-, ammonia; NH3).
  • True UV sterilization – works by electrically charging and breaking down oxidizers in the aquarium water. 
  • Water conditioners – these are very short-term/temporary reducers.
  • Naturally, reducers also get added to your fish tank from decaying organisms (plants and fish), fish food, and additives you may use in your reef tank. 

How To Test ORP In Reef Tanks

The most accurate testing method is the use of an ORP probe/sensor, measured in millivolts (mV). A consumer-grade ORP probe is the perfect choice for testing water samples like your reef tank. 

When using an ORP probe, it is important to regularly calibrate it to get reliable results. This is fairly straightforward with the use of an ORP calibration solution

Can ORP Measurements Be Inaccurate In Reef Tanks

Occasionally, ORP values taken in reef tanks may be inaccurate. The main issues come from factors that may interfere with the electrodes inside an ORP probe. 

There are four distinctive variables known to affect ORP measurements:

  • The pH level.
  • The water temperature.
  • The presence of cyanuric acid.
  • The phosphate concentration.

As a reef aquarist, you are strongly advised not to dwell on achieving “perfect” ORP readings, particularly if you have not recently calibrated your ORP probe with a calibration solution. 

The most useful way to optimize ORP is to look at the changes over time and note any drastic changes inside your reef tank.


If the oxidation-reduction potential (ORP) inside your reef tank is too high, you can lower it by adding metals, most organic molecules, and inorganic molecules, so they can give up electrons to oxidizers like oxygen molecules.
If you are unsure exactly which ORP device will best suit your needs, or you would like to learn more about other water measurements like pH levels and temperature in reef tanks, please do not hesitate to reach out to the world-class team at Atlas Scientific.

ORP Probes & Sensors

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