The saturation level of oxygen in the water depends on the temperature, specific gravity, and concentration of oxygen in the atmosphere over the aquarium. Aquariums maintained at high altitude will have slightly lower oxygen levels. For most reef aquariums within the normal specific gravity and temperature ranges given here the oxygen level at saturation is approximately 7 mg/L.
The oxygen level should be maintained close to saturation or a little higher. Supersaturation levels of oxygen are good to a point beyond which they are harmful. Harmful, high levels of oxygen can be reached within coral and anemone tissues as a result of photosynthesis (Dykens and Shick, 1984). They are not obtainable in an aquarium unless pure oxygen is administered under pressure in a contact chamber. Pressurized cylinders used for ozone contact, known as oxygen reactors, should never be used with pure oxygen.
since toxic supersaturated oxygen levels are easily reached with this kind of device (Burleson, 1989). Photosynthesis during the day elevates the oxygen level in the water above saturation. At night, respiration tends to reduce it below saturation. Circulation is especially important to keep the oxygen level from falling far below saturation at night. Good circulation prevents stratification of the water, and exposes the volume to the surface where gas exchange occurs. Protein skimming and surface skimming are also means of maintaining the oxygen level at saturation. Surface skimming removes the surface film that prevents gas exchange across the water surface, and exposes the thin layer of skimmed water to air as it swirls down the drain to the reservoir below the aquarium. Protein skimming removes compounds that would break down and consume oxygen (BOD and COD).
Oxygen testing is not necessary in the general monitoring of the aquarium water quality, since the level tends to remain within the same daily limits. It is lower at night when the plants aren't photosynthesizing, and higher during the day when they are. This also occurs in the natural setting on coral reefs. In aquariums, the oxygen level changes appreciably only in the event of putrification from a dead organism, or the fouling of the bottom substrate.
Redox is an abbreviation for reduction/oxidation, referring to types of chemical reactions. Oxidation and reduction reactions occur by electron transfer and by atom transfer. Reducing compounds or agents are electron donors, and oxidizing compounds or agents are electron acceptors. The process of oxidation involves a loss of electrons to oxidizing compounds, and the process of reduction involves a gain of electrons from reducing compounds. The transfer of oxygen atoms may also occur in redox reactions. Oxidation occurs with the gain of an oxygen atom, and reduction occurs with the loss of an oxygen atom.
In aquariums, redox potential is useful to measure because it gives a relative measure of the water purity. The measurement is a potential based on the sum of redox reactions occurring in the water. The higher the redox value, the greater the potential for oxidation to occur, the lower the number, the greater the potential for reduction. Reduction occurs in deep substrates, where natural redox values are negative. Measurements of redox potential in the ocean vary7 from 350-400 millivolts (Moe, 1989) to as low as 160-190 millivolts (Wilkens and Birkholz, 1986). Caution is advised in any
comparisons clue to differences in measuring conditions, technique and equipment used. Recommended aquarium redox levels range from 350-450 millivolts but each aquarist must go by the appearance of his/her own aquarium. Differences in probe placement, frequency of cleaning the probe, bioload, fauna composition, etc. all affect redox readings. It is not so much the value that is important, but the appearance of your aquarium inhabitants and the trend in measured redox values. If you notice j that the redox begins to decline rapidly, this is a sign that something is fouling in the aquarium and you should investigate the cause.
Aquarists should not forget that the appearance of the animals is the most important quality of the system. There is a common tendency for aquarists who have redox meters to become more concerned about the numbers on the digital display than the animals in the aquarium. It makes no sense to worry about redox numbers when the aquarium is fine. Don't aiin a good thing by striving for levels you hear are "best". Far more important is the pH and alkalinity7 level. A high redox level which might please the aquarist could occur with low pH and alkalinity, which does not please the corals.
Control of redox is achieved naturally through the techniques described in chapter 9. Artificial manipulation of redox is achieved through the use of ozone and a redox controller. See the topic ozone, this chapter for more detail.
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