REDOX (primer)

Discussion in 'User-Created Articles' started by jhnrb, Oct 8, 2006.

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  1. jhnrb


    Mar 9, 2005
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    Introduction to Redox Potential in the Aquarium

    The level at which the redox potential runs in your aquarium is not an "absolute" value that you must have, or your tank will not do well. It is really a "relative" number that can be used to monitor what is going on in your aquarium in terms of water quality. The later is, of course, affected by the chemical and biochemical reactions that take place in a tank.

    I usually aim for a number of 390 mv (millivolt), measured in the aquarium, and then watch what happens over time to that reading. A drop that persists would be a sure indication that something is going on in your aquarium that is not quite right. A stable number would indicate that the water chemistry itself is pretty stable as well.

    The suggested approach with regard to the number is to observe how the millivolt reading behaves. Does it go up, down, does it fluctuate often, or does it remain pretty stable within a certain range.

    Redox potential does not "have" to be 390 mv, or any other number, for that matter. I have seen tanks running at 320 doing real well too. On natural reefs though the value of the redox potential tends to be higher though than 320. Kipper (1989) related that measurements taken around the reefs of Western Australia varied greatly. Lows of around 250 mv were found in the morning. Highs were found around noon. Later during the day the millivolt reading would go down again. Overall though, the fluctuation would be steady in its amplitude (difference between the high and the low) and in the actual millivolt readings of those highs and lows.

    The ORP, as it is also referred to, would rise as the intensity of the sun, and the level photosynthesis increased. In some areas, on real sunny days, he measured values of over 500 mv. As the sun started to set, the ORP value would then start to drop as well. Median values around high noon though tended to be anywhere between 350 and 450 depending on how sunny a day it really was.

    The value is affected by the amount of photosynthesis (release of oxygen then takes place). The more oxygen released the higher the ORP will be. On real sunny days this means a high redox. On cloudy ones a lower one and on rainy days an even lower one.

    What the "value" of redox potential levels are though (and forgetting about what it really is chemically speaking), is that its level and trend (upwards or downwards) can give you an early indication that something is wrong in the tank.

    When you see a declining millivolt number you should be suspicious and start investigating what may be wrong.

    Look at the following example for instance: (all other factors are considered to remain the same)

    Day AM PM

    1 402 377

    2 403 376

    3 400 378

    4 380 355

    5 379 360

    6 377 356

    7 370 348

    8 368 332

    What you notice is that on day 4 the reading suddenly is lower, both in the AM and in the PM. This trend continues, and is actually accentuated on day 7 and 8.

    What this tells me is that on day 3 or 4 something happened in the tank and that the water quality is going down as a result of whatever it is that happened. This "event" let's call it, affects the oxidation reduction of the water downwards because pollution has been created and so the available amount of oxygen goes to work to deal with that pollution. As a result, due to the fact that oxidation is taking place, less oxygen is actually free in the water and the ORP falls.

    Viewed differently, because the water is no longer as pure, its ORP goes down. Initially the downward trend is slow but as the "event" continues to affect the water, the ORP will fall more rapidly.

    What event exactly happened I do not know, but that is exactly what I will set out to find out if I saw such readings. Indeed, finding the cause of the drop will allow me to correct whatever is wrong.

    The tank may still look good but the drop in ORP tells me that something is not right and I will take all measures necessary to find out what that is, and then remedy the situation as rapidly as possible. This is important and is the real benefit of using ORP "trend" readings. You can intervene before things actually get out of hand.

    In this respect ORP acts a a relative value that gives me an early sign of a possible problem and often warns you that something is at hand before things get out of control.

    What is it that I may find?

    Could be that a batch of algae died and lowered the water quality, or I may have lost a fish, or die-off is occuring on rock, or my skimmer is not functionning properly, etc.

    When this scenario happens we need to look carefully at all tank conditions and parameters and find out what is actually causing the ORP to drop and remain low.

    Note that, typically, the ORP will be high in the AM and lower in the PM because the pH in the AM is usually lower than in the PM. pH and ORP are inversely related. When one goes up, the other one goes down.

    I said all other factors remaining equal as indeed :

    if you add an animal the ORP will drop
    if you perform a water change it will drop too
    if you add additives in greater quantity than you normally do the ORP may drop as well depending on which one you use (with increased iodine it may rise for a while)
    Now you can also use the ORP, for instance, to determine if your tank is close to the capacity your filtration can handle. That is another way to use the relative value.

    Say your tank normally runs at 360 mv and you add another animal. The ORP drops to 330 but after about 24 hours it is back to 360. This means that the impact of the additional animal was neutralized and that your filtration was able to handle it.

    On the other hand, if you add that animal and the ORP drops and does not come back up, you would have an indication that you are close to a tank that is at capacity, and that you may need to add more rock, more sand or a different skimmer, or plainly that the tank is now overloaded and that you may need to consider a larger one.

    So, in short, the number allows for various ways of looking at what is going on in the aquarium.

    Of course we want the ORP to not fall below certain values regardless of what is going on but no one has demonstrated what that number is.

    Also, high values (usually over 500 or more) tend to affect the corals if they persist for too long. Corals close up or do not extend their polyps. The higher the ORP value the more they close. This is because the water is so oxidative that it is irritating to the corals - that is why when you add potassium permanganate, and you add too much and the redox jumps to say, 600 mv or so, all corals will close and will not reopen until it has dropped again.

    You can also watch the cycle happen using ORP values. When the tank cycles the ORP will be real low. As the cycle progresses the redox potential slowly starts to climb until it will max out at a certain number when the cycle is over and remain there, telling you in a way and without any other form of water testing that the cycle is over.

    Note that artificial redox levels maintained by ozonization are of less value. Indeed the ORP may drop because the amount of ozone being generated drops as well over time, and thus affects the reading in a downward fashion. Less ozone = less ORP.

    If you are using an ozonizer you can in fact determine from the ORP reading when the time has come to clean the unit, or change the bulb (depending on the type of ozonizer you have). Indeed, when the setting you have put in on your controller is no longer reached, or when the ozonizer has difficulty reaching it and remains on for most of the time, its output is obviously going down. This is an indication that you need to take care of the ozonizer.

    Although quite in fashion for a long time, and still effective, ozonizers are used very infrequently on reef aquariums. They are, however, still popular on fish-only tanks and on some real large reef tanks.

    Ozone generating devices tend to produce the high output they are rated for only for a short amount of time. After that the output drops and the impact of ozone on the ORP therefore diminishes. This can be remedied by cleaning the unit, or if need be changing the ozone generating bulb.

    Adding hydrogen peroxide (not at all recommended, but some hobbyists do) will make the redox potential drop to the 70 or 80 mv level (actually one good use of it is when one injects ozone and has residual ozone in the tank - adding just one drop of hydrogen peroxide will neutralize the residual ozone instantly).

    jhnrb, Oct 8, 2006
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