Although it is difficult, and probably not appropriate, to select one water quality parameter as being more important than any other one, dissolved oxygen levels certainly qualify as one of the most crucial ones for the survival of all lifeforms in the tank. Indeed, its level conditions the ability of all fish, invertebrates, corals, bacteria, etc. to survive and thrive. Its level reduces the stress on the fish etc., and its level ensures that a sufficient amount of oxygen is available for proper biological filtration.

Oxygen levels are measured using standard tests sold by several companies. Most of these tests are based on the Winkler test, dating back to the 1880's, and small modifications of that test (for example the LaMotte Chemicals test). One can, however, use a sophisticated instrument to do so as well. Dissolved oxygen meters are expensive instruments and really not something the average hobbyist is really in need of, especially now that at least four aquarium product manufacturers offer a chemical test that is reasonable in price, easy to use, as well as highly accurate.

Should you, nevertheless, decide to acquire such a meter, you should buy a unit that is fully calibratable, and preferably adjustable for measurements in salt water (higher chlorinity level). Such a unit, and the necessary electrode, may well cost over $800.00. Because the level of dissolved oxygen saturation is different in salt water than it is in fresh water, the ability to adjust for chlorinity is important. It will ensure a more accurate reading and prevent you from having to refer to charts to calculate the true D.O. level.

Often when dissolved oxygen levels are low, the tank will be in less than perfect shape, and all animals will be stressed. You must always determine the cause for this oxygen deficit, and remedy the situation by correcting whatever problem was causing the oxygen to drop. Not doing so will lead to more problems and probably outbreaks of parasitic disease, and invertebrates that do not open to their fullest. When your tank does not look all that good, suspect lower dissolved oxygen levels and do what is required to bring them back up.

4.9 CARBON DIOXIDE DIFFUSION EQUIPMENT:

CO2 is a very common gas that is used for two main purposes in both freshwater and marine aquaria: fertilization of plants and macro-algae, and balancing the pH of the water. Not all hobbyists who maintain reef tanks maintain macro-algae in their tanks as well, but most hobbyists who maintain a reef tank and run it with a trickle filter, may need to add CO2, especially if they maintain the water at a high degree of carbonate hardness. Such high levels are advocated for the benefit of corals (Thiel 1986).

Change one, and at least one of the other two will be affected as well. This is important, as by influencing this equilibrium, one can also influence the amount of carbon dioxide that is dissolved in the water.

Because CO2 is important for the formation of calcium carbonate in the artificial seawater in our tanks, we must ensure that sufficient quantities of it are available. Indeed, calcium carbonate is important to all corals kept in our tanks. In fact, adding supplemental calcium carbonates, in the form of Kalkwasser is highly recommended.

Carbon dioxide can be added in two ways: manually and in automated fashion. Both are viable solutions for the reef hobbyist. Manual systems are, of course, far more affordable than automated ones that cost a goodly more. Often hobbyists will start with a manual setup, and upgrade the latter to an automated one as time goes on. Indeed, all parts bought for a manual setup can be reused and are not redundant when upgrading. Manual systems are easy to set up, affordable, and can deliver the same benefits as the automated ones, providing they are set up correctly.

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