Ozonation in the reef aquarium

Ozonation in the reef aquarium

Ozone is one of the most contested tools in the aquarium hobby. Some hobbyists swear by it; others consider it an unnecessary risk. The truth is more neutral: ozone is an effective water-treatment method that works well when you understand what it does — and what it does not do.

In this article we cover ozone chemistry, equipment, ORP control, trace element interactions, safety and the most common myths. The article is aimed at a hobbyist who wants to understand the mechanisms rather than simply following numerical rules.

What ozone does in water

Ozone (O₃) is an allotrope of oxygen (O₂) — the same element, but with three atoms instead of two. This extra atom makes ozone a powerful oxidising agent: it donates its surplus oxygen atom to almost any organic molecule it encounters, then breaks back down to O₂ without leaving its own residue in the water.

Ozone’s reactions are not random. The molecule reacts selectively: it attacks carbon–carbon double bonds, aromatic structures and other reduced functional groups first. In practice this means that complex organic molecules — for example the breakdown products of coral mucus secretions, fish metabolism and food remnants — are cleaved into smaller intermediates: aldehydes, ketones and carboxylic acids.

This is the core mechanism of ozonation, which is often misunderstood: ozone does not remove dissolved organic carbon (DOC) from the water. It changes the nature of DOC. Refractory compounds — structurally complex molecules that resist microbial decomposition — are converted into labile intermediates that microbes can utilise more readily.

The practical consequences are:

• High-molecular-weight DOC is cleaved into smaller fractions
• The oxidation state (redox state) of the water rises
• CDOM — chromophoric DOC that absorbs light and causes the water to yellow — is reduced
• Heterotrophic bacteria receive more easily metabolisable substrates, grow faster and convert dissolved carbon into particulate biomass
• The skimmer removes this biomass more efficiently than the original refractory DOC

The cycle works like this: ozone conditions → bacteria consume the conditioned carbon → biomass grows → skimmer removes the biomass. Dissolved carbon is difficult to remove mechanically. Carbon that has been converted into microbial biomass is not.

There is also an important side effect: as ozone oxidises reduced organic compounds, chemical oxygen demand (COD) decreases and the redox balance shifts towards a more oxidised state. This shows up on an ORP meter as a rising reading — but ORP is only an indicator, not a direct measurement of ozone concentration.

Corals are surrounded by a thin diffusive boundary layer where water exchange slows near the tissue surface. If DOC accumulates and accelerates microbial growth, a localised oxygen deficit can develop in this microenvironment overnight, even when the tank’s general oxygen level appears normal. Ozonation that keeps the DOC cycle running smoothly therefore affects coral wellbeing beyond simply improving water clarity.

ORP — what it measures and what it does not

ORP (oxidation-reduction potential) is a voltage measurement in millivolts. It indicates how much oxidising or reducing capacity is present in the water relative to the other. A high ORP means a more oxidised water column; a low ORP means a more reduced one.

Natural seawater ORP typically ranges between 350–370 mV. In a reef aquarium with a skimmer, live rock and good circulation, the same range is achievable without ozone, but in heavily fed tanks or those with a large bioload ORP can sink to the 200–300 mV level.

Target range during ozonation: 300–450 mV. A value above this is a sign of overdosing or sensor malfunction.

An ORP probe does not measure ozone concentration directly. It measures the overall redox state of the water, which is influenced by ozone as well as pH, dissolved oxygen, ammonium and nitrite levels, CO₂ and organic load. The same ORP reading can be achieved by many different chemical combinations. Readings from two different tanks cannot be directly compared.

Practical limitations:

• ORP probes age: an oxide layer forms on the platinum or gold electrode surface, slowing response time and causing drift
• Calibration is performed monthly with a known standard solution (e.g. 475 mV or 200 mV standards)
• If the reading stays completely unchanged for more than 24 hours, jumps by more than 50 mV without cause, or deviates more than ±50 mV from the standard, the probe is faulty or its calibration has failed

ORP is a useful indicator of the tank’s general condition — Aslett’s (2024) visual diagram illustrates well how visible water clarity and ORP reading correlate. But it should not be read like a thermometer: the absolute number matters less than the direction and stability.

Practical benefits

The benefits of ozonation documented in the literature are:

Water clarity. CDOM is a fraction of DOC that absorbs light particularly in the blue wavelength range. A high CDOM concentration makes the water appear yellowish or grey and reduces the blue light reaching the corals, which is the most important wavelength range for driving photosynthesis. Ozonation breaks down CDOM rapidly and visibly improves water transparency.

Skimmer efficiency. Ozone acts via two pathways. Some of the oxidised organic fragments become more surface-active and adhere to air bubbles more readily. Simultaneously, dissolved carbon converted into growing microbial biomass becomes particulate matter that the skimmer can mechanically remove. The combined effect of these two pathways is that skimmate produced by the skimmer darkens and becomes denser.

Pathogen reduction. Ozone kills free-swimming bacteria, viruses and free-swimming stages of parasites in the water column. This reduces the general titre of pathogens in the tank, even though it does not replace quarantine or treat parasites already attached to fish.

The microbiome responds too. There is a caveat here: research data shows that ozone is selective. In the Beyond the Reef podcast, Salem Clemens stated, based on AquaBiomics data, that ozone specifically breaks down aromatic phenolic compounds and is effective against microbes that operate through these compounds — including some beneficial bacteria such as Pelagibacter. Short-duration, ORP-controlled ozonation is a different matter from continuous, high-dose use. In dosing, less is more.

Equipment

Ozone unit

Two technologies are used at the hobbyist level:

Corona discharge is more powerful: the discharge ionises air molecules and produces O₃ at high concentration. Best suited to larger and heavily fed tanks. More adjustability, higher purchase cost.

UV ozone unit produces O₃ via short-wave UV radiation. Smaller capacity, but sufficient for compact systems. Simpler construction, does not require a dryer as critically as a corona discharge unit.

Dosing guideline: 5–25 mg/h per 100 gallons (approx. 380 litres). Always start at the low end of the scale and adjust upward based on ORP — not on a schedule.

Dryer (mandatory)

Humidity has a significant effect on corona discharge unit performance: moist air reduces ozone output and shortens the unit’s service life. A dryer removes moisture from the air entering the unit before it reaches the reaction chamber. For UV units a dryer is recommended, though not as critical.

Reactor vs. skimmer injection

Ozone can be introduced to the tank in two ways:

Skimmer injection: ozone is fed into the air intake of the skimmer. Practical, because the skimmer already processes water and an activated carbon stage downstream is easy to add to the return line. Requires an ozone-resistant skimmer — standard plastics and rubbers degrade under ozone.

Ozone reactor: a dedicated contact chamber in which ozone and water are in precisely controlled contact. More precise control, but requires additional space and investment. Activated carbon at the reactor outlet is always mandatory.

Materials

All tubing and hose in contact with ozone must be made from ozone-resistant material — silicone hose is standard. Standard PVC or rubber hose degrades chemically and the breakdown products end up in the tank.

Activated carbon — mandatory safety measure

Activated carbon at the ozone outlet is not optional. It is a non-negotiable safety measure.

Ozone does not fully react before reaching the display tank. Residual ozone — the portion that did not react in the reactor or inside the skimmer — can cause oxidative stress directly to corals and invertebrates. Activated carbon absorbs residual ozone and also secondary oxidation products (SOx) that form particularly in bromide-containing salt water.

Replacement interval during ozonation: 1–2 weeks. This is substantially shorter than the normal 4–6 week cycle, because the activated carbon surface is inactivated more rapidly under a high oxidant load.

Trace element interactions — special attention to iodine and bromine

Ozone is a powerful oxidiser that changes the chemical state of certain trace elements and thereby their bioavailability. Two elements require particular attention.

Iodine

Iodine occurs in the reef tank primarily as iodide (I⁻), and is often dosed as organic PVP-iodine (polyvinylpyrrolidone-iodine), which releases iodine slowly and in a controlled manner. Under normal conditions this is exactly what is wanted.

When ozone is present the situation changes. Ozone rapidly oxidises PVP-iodine to free iodine (I₂) and further to iodate (IO₃⁻). Free iodine is significantly more reactive than the bound form in PVP-iodine. A sudden spike of free iodine can cause coral bleaching, polyp retraction and tissue necrosis.

Practical measures:

• Reduce PVP-iodine dosing during ozonation
• Monitor iodine levels regularly (target 0.03–0.06 ppm)
• Keep ORP below 400 mV when dosing iodine

The distinction matters here: PVP-iodine is a slow-release form; free iodine is the reactive form. These are not the same thing, and confusion at this point leads to errors in dosing decisions.

Bromine

Ozone can oxidise bromide ions (Br⁻) to bromate (BrO₃⁻). Bromate is a toxic compound. At natural seawater bromide levels this is not generally a significant risk, but in two situations it can become a problem:

1. If the salt mix used contains a higher than normal bromide content
2. If two-part dosing has been used to raise bromide levels

Symptoms: polyp retraction in corals, abnormal fish behaviour, slowed growth.

Practical measures:

• Perform regular water changes to dilute bromide levels
• Keep ORP below 400 mV
• Monitor bromide levels in ICP analyses

Safety

Ozonation is safe when implemented correctly — but mistakes produce adverse effects quickly. The following principles are non-negotiable.

ORP controller is the primary safety device

Without an ORP controller, continuous ozonation almost certainly leads to overdosing. The controller measures ORP and switches off the ozone unit when the set upper limit (recommended maximum 450 mV) is reached.

Additional safety: mechanical timer. The ORP controller probe can fail — in which case the unit may run continuously. Install a separate timer in the ozone unit’s power circuit that limits run time to, for example, 20 minutes per hour regardless of what the controller commands. These two complementary safety systems together guard against catastrophic overdose.

The sense of smell is sufficient as a warning

Ozone smells strongly of a chlorine-like odour — it is highly recognisable. If ozone is detected in the aquarium room, it means gas is escaping into the water and evaporating from it into the room air. This is an alarm requiring immediate action. At high concentrations ozone damages the respiratory tract.

Action: switch off the ozone unit immediately, improve room ventilation, check activated carbon and the reactor/skimmer injection.

Symptoms of overdose

Emergency protocol in the event of an ORP spike

1. Switch off the ozone unit immediately
2. Maximise aeration: run the skimmer wet at full air, increase surface movement
3. Perform a 20–30 % water change
4. Add fresh activated carbon to the flow zone in the sump
5. Monitor animals closely

If animals are suffering severely and immediately and the situation does not stabilise, sodium thiosulphate (Na₂S₂O₃·5H₂O) can be used as an emergency neutraliser: dissolve ¼ teaspoon in 250 ml of RO/DI water and add slowly to the sump flow zone. This is a last resort for an acute emergency — not a routine action.

Connection to DOC dynamics

Ozone’s operating logic is easiest to understand in relation to the tank’s DOC cycle. In a closed reef system DOC does not disappear on its own — it accumulates unless actively managed. In a natural reef the ocean continuously dilutes it, but in an aquarium that dilution does not exist.

The DOC problem in a tank is not simply a high concentration — it is imbalance: when carbon input exceeds the processing and removal capacity, the reservoir shifts towards slower, refractory fractions. These do not break down quickly; their accumulation changes microbial behaviour and slows nutrient cycling.

Ozone corrects precisely this point: it converts refractory compounds into labile intermediates, allowing bacteria to utilise them, grow faster and become biomass that the skimmer can remove. The result: turnover rate increases, accumulation of slow fractions decelerates, skimmate improves.

Ozone does not, however, replace water changes. It does not replenish trace elements, it does not remove nitrate and it does not maintain pH and alkalinity. These require their own management mechanisms.

When ozonation is justified

Ozonation is a tool worth considering in the following situations:

• The tank is heavily fed or the bioload is large, and DOC accumulates despite other export methods
• Water clarity is a problem despite a well-functioning skimmer and a good water-change schedule
• Supporting a general reduction of pathogens as part of broader water quality management is desired
• A larger closed system (particularly public aquariums) where DOC cycling is critical

Ozonation is not justified in the following situations:

• The tank is small and well managed with water changes — the added complexity of ozonation brings no extra benefit
• The skimmer is not working properly — fix the basic equipment first
• The goal is to treat parasites attached to fish — ozone cannot do this
• An early-stage tank whose microbial community is still establishing

Myths

“Ozone kills beneficial bacteria.” Not true as a blanket statement. Nitrifying bacteria live on surfaces — in live rock, sand, bio-media. Correctly dosed ozone does not reach these surfaces. Overdosed ozone can disrupt nitrification temporarily, and this is one reason why ORP control is mandatory. It is also true that certain free-swimming beneficial bacteria — such as Pelagibacter — can decline as a result of ozonation (Salem Clemens, AquaBiomics data). This is a different claim from the destruction of nitrifying bacteria.

“Ozone replaces water changes.” Wrong. Ozone does not replenish depleted trace elements, it does not remove nitrate and it does not maintain pH or alkalinity. Water changes remain essential.

“Ozone is a dangerous substance.” Used correctly — with ORP control and into activated carbon — it is not. The danger arises from overdosing or inadequate filtration.

“Any skimmer will work with ozone.” Wrong. Only skimmers made from ozone-resistant materials are suitable. Standard plastics and rubbers degrade under ozone — the breakdown products end up in the tank.

“Ozone removes all parasites.” Wrong. Ozone kills free-swimming stages of parasites in the water column but not trophonts attached to fish or encysted forms. It does not replace quarantine.

“Ozone clears the water immediately.” Wrong. Improvement happens gradually. If water turbidity is caused by an algae or bacterial plankton bloom, the underlying nutrient problem must be addressed — ozone alone will not resolve it.

Parameter reference values

References

1. Peer-reviewed studies

• Aguilar-Alarcón, P., et al. (2022). Impact of ozone treatment on dissolved organic matter in recirculating aquaculture systems. Water Research, 215, 118263.
• Gregersen, K. J., et al. (2021). Foam fractionation and ozonation in freshwater recirculation aquaculture systems. Aquacultural Engineering, 93, 102158.
• Hansell, D. A., & Carlson, C. A. (2015). Biogeochemistry of Marine Dissolved Organic Matter. Academic Press.
• Haas, A. F., et al. (2016). Global microbialization of coral reefs. Nature Microbiology.
• Kovács, B. D., et al. (2023). Evaluating protein skimmer performance in a marine recirculating aquaculture system with ozonation. Aquacultural Engineering, 103, 102381.
• Perrins, J. C., et al. (2006). Secondary oxidants in ozonated ballast water. Marine Pollution Bulletin.
• Westerhoff, P., et al. (1998). Relationships between the structure of natural organic matter and its reactivity towards molecular ozone and hydroxyl radicals. Water Research, 33(10), 2265–2276.

2. Hobbyist literature and brand documentation

• Dus, A. (2025). Ozone, Bacteria, and the Microbial Loop — What Really Happens to Dissolved Organics in a Reef Tank. Avast Marine / Manta Systems Labs.
• Marshall, T. (2025). Mastering Ozone in Saltwater Aquariums — Complete Guide. Manta Systems.
• Aslett, C. G. (2024). Teach a Person to Fish… SPS Academy Part V. Reef Ranch. https://www.reefranch.co.uk/
• Southernland, A., Dank, K., & Clemens, S. (2024–2025). Dissolved Organic Carbon — podcast transcript, Beyond the Reef / Fauna Marin.
• Aslett, C. G. (2024). Fundamental and Foundational Science: Not Simply Water (Coral Nubbins supplement). Reef Ranch.

3. Literature and textbooks

• Delbeek, J. C., & Sprung, J. (1994). The Reef Aquarium: A Comprehensive Guide to the Identification and Care of Tropical Marine Invertebrates, Vol. 1. Ricordea Publishing.
• Balling, H. W., Janse, M., & Sondervan, P. (2008). Trace elements, functions, sinks and replenishment in reef aquaria. In Leewis, R. J. & Janse, M. (eds.), Advances in coral husbandry in public aquariums. Burgers’ Zoo, Arnhem.
• Munn, C. B. (2019). Marine Microbiology: Ecology and Applications, 3rd ed. Garland Science.