Electrodes can be placed in the water just about anywhere it is convenient for your particular setup. They do not need to be in a specific location. Once you decide on one, however, leave it there. Such will allow you to keep accurate records being you are always measuring the redox potential in the same location. As already indicated the ORP is influenced by many factors. To get a stable reading you should not keep moving the electrode around.

Here are a few factors that you must be aware of in order to be able to interpret your own tank's redox accurately, and decide where to place your probe (read them carefully and decide which ones will apply to your tank):

• pH and redox potential are inversely related. When the pH level goes down, the redox potential goes up, and vice-versa. 1 pH degree = 59 millivolt.
• The level of metabolism taking place in the tank changes the redox and reduces it as the rate goes up. This is also an inverse relationship. This is the reason that your redox potential will have a tendency to go down as soon as the lights come on. When the lights come on, metabolism increases. This can also be expressed as time sensitivity. Do not compare a redox reading taken at 9.00 am one day, with one taken at 2.00 pm on another day, or even on the same day. Because the metabolic processes are different your readings will be different as well.
• Carbon dioxide will influence the reading you obtain, especially if your redox electrode is placed too close to the area where the water/carbon dioxide mix re-enters the tank, or the sump of your filter.
• Raising the buffer will affect the redox potential reading downwards. Because a high buffer is necessary for corals and for pH stability, such is a fact you will have to live with.
• Different types of electrodes will give different readings for the same water, in the same spot, at the same time of day. The two main types of electrodes are Platinum/silver/silver chloride and Calomel. Use only the former in a reef tank. Calomel electrodes contain mercury.
• Because ozone has a very high redox potential, never place an electrode too close to the area where the water from your skimmer, or ozone reactor, is re-entering the sump of your filter. Your reading will be much higher than it really is, if you do.
• Changing the stocking level of the tank will affect the redox potential. Indeed as you increase the load the filter has to deal with, the water quality will suffer somewhat, and your redox will be a little lower as well. If the system does not go back to the level it had before you added those animals, you are getting close to having an overloaded tank. If you add several animals at a time this drop can be significant.
• Changing water and adding top-off water will affect the redox potential for a while, because by adding new water (and salt in the case of a water change) you are changing the chemical composition of the aquarium's water. Your electrode will reflect that, especially since newly prepared salt water has a lower redox potential.
• Feeding, which is really adding a lot of organic material to the tank, will lower the redox. The system should, however, come back to its original number in a matter of 1 to 2 hours. If it does not, you have a highly loaded tank.
• Trend analysis of your redox potential is very important. When comparing values taken over a period of several days, or weeks, a trend emerges. Comparing numbers taken over a period of weeks, but only the ones taken at the same time of day, gives an even better idea of the water chemistry of your tank. Example: if your redox is showing a small but constant increases in values for all redox potential levels measured at 9.00 a.m., your tank is obviously doing well, and the water quality is in fact improving slowly.
• As the temperature of your tank rises, its redox potential will go down, and vice-versa. This is similar to its dissolved oxygen content. Yet another good reason to keep the temperature of the tank within the recommended ranges.
• Redox potential electrodes accumulate dirt and small algae. This causes the readout to be distorted, either upwards or downwards. Clean your electrode regularly as suggested in the maintenance and husbandry chapter. Upward distortions occur when micro-algae grow on the tip. When they release small amounts of oxygen, as a result of photosynthesis, a higher than actual redox will be displayed. When plain dirt settles on the tip, and prevents water from touching it, a lower than actual redox is the result. Either way, the reading will be erroneous.
• Many redox potential electrodes react much slower to changing conditions than, for example, pH electrodes. This is especially so the first time you use a new one. Since the cap that is placed over the tip may have been on the electrode for quite some time, the tip itself is saturated with whatever solution was in the cap. This is the same solution as the one inside the electrode, more often than not it will be silver chloride. When you now place the electrode in the tank's water, quite some time can go by before the super saturated tip rids itself of the silver chloride and starts reading the actual redox of the water in your tank. Ergo the long time before you get a reading that is accurate the first time around.
• Always place the electrode in an area where good water circulation exists. This is important as in doing so the water passing by and over the electrode sensing end will always be representative of the water in your tank.
• Since CO2 lowers the pH, and since lower pH levels are associated with higher redox levels, it is not a good idea to place the electrode too close to the effluent of a carbon dioxide reactor.
• Since ozone has a redox potential of over 2700 mv, you should not place the electrode too close to the effluent of a skimmer or an ozone reactor. Your reading would be much higher than actual.

Redox potential can also be defined as the ability of the water in your tank to deal with the pollution of all kinds present in that same tank. The higher the redox the better it can deal with such pollution rapidly and efficiently. Chemically speaking such is not a good explanation of redox potential, but for our purpose it is accurate and suffices.

The higher the redox value, the better the water can deal with pollution and in the process the redox will be lowered. If the redox value is high there can not be a lot of pollution, because if there was, the redox would no longer be high, as it would have been "used" up in the process of dealing with that same pollution.

High purity water = water with a high redox = less stress on the life forms = a better looking tank and more vibrant looking animals. If the redox potential is low, decay must be taking place because decay and its by-products lower the redox potential and the quality of the water. To keep the redox potential high we must, therefore, eliminate all sources of pollution. Any process that involves reduction of organic material lowers the redox potential even more rapidly.

Below are some reasons for low redox levels (and the list is not exhaustive either):

• Over feeding or feeding too frequently, especially feeding repeatedly during the same day.
• Dead material in the tank, whether it be algae or fish or small animalcules etc.
• Dirty filters, especially dirty mechanical filters.
• Clogged undergravel filter areas. If you still use such a filter, make sure you clean it regularly.
• Clogged or low oxygen areas in layers of substrate that are too thick.
• Dirty pre-filters in trickle filters and siphon boxes.
• Spent activated carbon that has not been changed and has picked up a lot of detritus.
• Dirty sponge material in filters, canisters, and so on.
• Unclean corner overflows.
• Over crowding of the tank. Too many animals tax your filters.
• Too small a trickle filter.
• Unsuitable medium in the biological filter.
• Low dissolved oxygen levels.
• Nitrite in the water.
• Denitrators that are not operating as they should.
• Excessive amounts of liquid foods.
• Using too much nutrient additives that are high in organic components.
• Salt of low quality that does not contain all required chemicals.
• Salt that contains too many impurities.
• Certain fertilizers, trace elements, and so on.

*...

Many reasons indeed, and all can be controlled by the hobbyist who is willing to take the time and make the effort to take care of his or her tank according to a regular schedule of maintenance and husbandry, such as the one described, for example, in Chapter 9.

There are still other reasons, more acceptable, or normal ones, that you need to accept as a fact of life, because they are, in most cases, unavoidable:

• Whenever you perform a water change you are adding water of a redox potential that is the range of 220 to 240 mv. Indeed, freshly prepared saltwater, and saltwater that has been standing around for 24-36 hours has a redox that is relatively low. This water of a lower redox will lower the redox of the tank water for a short period of time.
• When you feed, and you should, but not excessively, the redox will go down. Perhaps by as much as 60 to 75 mv. The original level should come back within 1 to 2 hours at the maximum.
• Fresh unused activated carbon can lower the redox for a short period of time. So will other products that you need to use. Some additives, especially when you add a large amount of them, rather than dosing them with a pump, will reduce the redox for a few hours.