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Related FAQs: Reduction Oxidation, Ozonizers 1, Ozonizers 2Ozone 3, Rationale, System/Selection, Application/Installation, About Dryers, MeasuringMaintenance/Repair, & Marine Test Gear 1, Marine Test Gear 2, Marine Water Quality, Marine Water Quality 2, Marine Water Quality 3,

Related Articles: Reduction Oxidation Potential, RedOx: A Very Valuable Tool For Assessing, Assuring Maine Aquarium Health, ppt. presentation, Part 1, Part 2, Part 3, by Bob Fenner, Marine Water Quality, Maintenance, Marine Filtration, Mechanical & Chemical, Ozonizers, 

/The Conscientious Marine Aquarist

RedOx; Reduction-Oxidation Potential, ORP

By Bob Fenner

Give me high RedOx or give me death.

What, another strange concept like love, pH and the "deficit" to contend with? Not necessarily. What did aquarists do before the popularization of this measure of water quality? Did their tanks turn into bouillabaisse? No; then again their successes, and knowing the reasons for their failures were more limited.

Re-Dox (Re as in "real", Dox as in wiener dogs) like pH, can give valuable insight as to change in the suitability of your water, particularly with "reef set-ups" whose organisms are more RedOx sensitive.


RedOx is short for Reduction Oxidation. Having taught high school level chemistry, I pride myself in being able to describe bizarre phenomena to wide (interest and background) audiences. Let's try a few definitions.

RedOx is a measure of a systems capacity to oxidize material. Defining terms with other unknown abstracts doesn't help much. Put another way, RedOx is a classification category of chemical reactions. The helpful acronym OILRIG, Oxidation Is Losing, Reduction Is Gaining; losing and gaining what? Electrons. You may remember from Science classes that the bulk of chemical interactions around us are proton donor/acceptor (sometimes called acid/base) in nature. RedOx reactions are fewer and slower by comparison. In reduction-oxidation reactions one chemical species loses, another gains electrons.

RedOx potential is a value (number) defining how much gaining/losing a system might do; an oxidizing environment (one's we want) has a positive value.

Biological Importance:

I like John Tullock's definition of RedOx offered in "The Reef Tank Owner's Manual" ...a measure of the ability of a system to eliminate wastes". Not totally accurate scientifically, but very valuable as a subjective description.

RedOx is eminently important as an indicator not only of a system's capacity for cycling waste, but indeed of chemically supporting fish, plant, and invertebrate life. There are both oxidation (e.g. biological conversion of ammonia to nitrites to nitrates) and reduction (ridding systems of nitrate aka denitrification, bio-phosphate PO3) that must occur readily in a truly closed system to support (macro-) life.


RedOx is measured indirectly as the ability of an aquatic system to conduct electricity, in millivolts (mV, 1/1000 of a volt). Hence the term RedOx potential, or, if you will, ORP for, you guessed it, Oxidation-Reduction Potential.

Testing can be done periodically, but ORP can change so rapidly that in order to fully benefit, most people who are so inclined, monitor it on a continuous basis.

Take a look at the hobby magazines and the several "reef" tomes. There's seemingly no end to the ads and infomercials promoting the special devices called RedOx monitors and controllers. What a boon for the electronics trade.

These digital display units are attached to a probe that you place in your system or filter reservoir. Millivolt data may be just displayed, recorded, sent along to a fancy doser to slurp enhancing-enhancing fluid into your system, or to a switching-mechanism to turn off/on your ozonizer.

"Good" RedOx Values:

Can you get too much of a good thing like RedOx? Definitely. RedOx potentials above 400 mV are dangerous to life. Ideally you want a range of @ 350-390 mV. Most properly set-up and maintained systems display an average of 300-340 mV, some vary down to the 200's but most writers agree, values below 300 mV are to be avoided.

RedOx fluctuates in the wild and in captive systems throughout the day. As respiration and photosynthesis increases/decreases RedOx tends to drift downward during the day, reversing the trend at night.

Hopefully, from all this you can see that RedOx values by themselves are worthless. Drain cleaner and bleach solutions have very high RedOx values. Would you add them to your system?

To me the real worth in RedOx (and actually all test gear values) is being able to tell whether something is amiss, or about to be so.

Knowing what your systems usual ORP signature is during an interval, seeing a sharp (@50 mV) or steady (5-10 mV) change per day should lead you to further investigation (and reaction). IS there "something" dead/decomposing in the system? Did a "little helper" pour in a whole packet of food? Is your protein skimmer on the skids?

Reserve/Resistance to Change:

Here again, as with pH and alkaline reserve we are talking about the various influences that serve to produce a positive reduction-oxidation potential and those which tend to drive it down. Natural and artificial seawater displays a resistant to downward shifting of both. Captive systems, being overcrowded and overfed lose RedOx potential with accumulation (and reduction) of wastes, loss of dissolved oxygen (usually transient), respiration, proliferation of microbes....

As with pH, the aquarist wants to promote activities that add to oxidation potential, and stave off reductive effects within reason.

Can You, Should You, How To Change RedOx:

Yes, probably not, read on. Yeah you can definitely do several things to raise and sustain high RedOx. Most all of these are "just" common sense proper installation and maintenance items. Should you react to/alter any given value? Not really, unless you see a precipitous drop within one day, or a steady loss over a several days, there really is probably nothing wrong with your system. RedOx can be altered upward, easily.

Oxygen sounds a lot like oxidation for good reason. A great

deal of measurable oxidation potential is due to oxygen concentration. Oxygen levels below saturation? Change your airstones, crank up the flows, ORP will rise. The biggest Oxygen high can be gotten from the use of ozone (O3P), and the most dangerous. Misused (too much, improperly with a poorly engineered protein skimmer) Take care if utilizing this strong oxidizer; ozone is probably the biggest culprit of pegging a system over 400 mV. It can kill.

Looking for a "natural" high? Try algae (e.g. Caulerpa), adequate amounts of "live rock", to some degree "live sand".

Removing organics through tuning up your skimmer, chemical filtrants, cleaning filter media, vacuuming substrates... reduces reductive influences, effectively raising ORP. Dittos for cutting back on feeding or bio-load (under-crowding).

Really desperate (this is intended to be "tongue in cheek")? Lowering temperature will increase gas solubility. The increased oxygen concentration will raise ORP, as will reducing the metabolism of your livestock. You could drop your pH (do not do this, I'm only trying to make a point), move your probe to an area with more brisk circulation, or even get a new one. Even new probes are notoriously variable in their within group readings.


Is it absolutely critical to know what RedOx is, measure and manipulate it to be a successful marine aquarist? Do you need to understand the nature of carbon-carbon bonds and what 'octane' means in order to burn gas in your car? No to either.

If you have the money, interest and patience to look beyond such standard test as pH, temperature, specific gravity and nitrogen processing, RedOx is on (of very many) that can grant you insights into what's "going on in your water", and what you "might do" to incrementally improve conditions.

Meters, Dosers and switches cost real money (a few to several hundred dollars), require calibration and some maintenance. This might be "peanuts" compared with the livestock and emotional investment you have in your system. You'll have to decide for yourself whether this expensive "tool" fits your philosophy and "tool-box".

Despite all the seeming statements to the contrary you do want a higher, as opposed to lower, ORP. Higher oxidation potential accelerates oxidation of ammonia to nitrite to nitrate, conversion of organic PO3 to inorganic phosphate, PO4.

Bibliography/Further Reading:

Paletta, Michael. RedOx revealed; it's nothing more than a tool. AFM 11/95.

Prasek, Edward D. 1994. Marine Aquarium Equipment; are you getting the most out of yours? AFM 10/94.

Prescott, Shawn. 1994. Water chemistry in marine aquaria; with special emphasis on RedOx, and its interactions with nitrification, denitrification, protein skimming, ozone, and more. FAMA 9/94.

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