Did you say ORP?

Staying on the ORP topic, lets take a look at what it really means. It would be unfair to talk about using ORP as a measure of your tank’s ability to convert and consume wastes without properly diagnosing what it really is.

ORP is short for oxidation – reduction potential. Or Redox for some. Measured in milliVolts (mV). Say what?

Since I am not a science guy, or nor do I pretend to be one, lets allow someone who is qualified to explain what it means:

A REDOX reaction has two parts.

• Oxidation – an atom loses one or more electrons, he’s just been oxidized, and his oxidation number increases
• Reduction – an atom gains one or more electrons, this guy’s been reduced, and his oxidation number decreases

(taken from reefs.org, a great article by Eric Edelman)

So how does this work in the fish tank?

REDOX reactions are going on in your fish tank all the time. Many different types of reactions such as synthesis, addition, combustion, and decomposition reactions can all be additionally classified as REDOX Reactions.

It also happens that most of the common elements and compounds that are in our tanks such as calcium, sodium, chlorine, bromine, etc… all undergo these type of reactions. Many of these reactions provide our reef inhabitants with the different compounds they need to grow and live.

So what numbers (in mV) are ideal for my tank  What should I strive for?

The answer is that they shouldn’t really BE anything. ORP values vary from tank to tank, and they even vary within one single tank.

The best indication ORP is the trend that they have, that is where all the important obtainable information is. Some general ideas, or rule of thumb, on where your ORP value should be. 200 mV is pretty low. 500 mV is pretty high. My own personal tank ran at around 375 in midafternoon. When it dropped to 325 or so, I’d clean the skimmer out, and the ORP would rise again in a few hours. Good skimming is the BEST way to maintain solid ORP values. ORP correlates with dissolved oxygen, and inversely correlates with your pH.

So as the pH goes up, the mV values in your ORP reading go down. I guess we can assume that bigger skimmers, great gas exchange, and photosynthesis (all great oxidisers) raise ORP values and lower pH values. And inversly, poor skimmers, and low gas exchange will lower ORP and raise pH.

So is there a ‘normal’ mV number that we should strive for? (Bob Fenner wrote):

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.

Also,  as Bob Fenner put in in his WetWebMedia article:

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 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.

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.

Exactly my thoughts Bob. You shouldn’t use these figures as the sole measure on how your tank is doing. It should be used in correlation with many other readings that you get from your meters. Not only that, you can get most of what you need from just looking at your tank. But it sure helps to know if something is amiss.