Oxygen Saturation
I am trying to find out why, and at the same time, figure out what the saturation level measurement would be.
That depends on the exact salinity and temperature of your aquarium -- the table below gives you the values of oxygen saturation (mg/l) across a wide range of temperatures and salinities. The basic answer is that saturation for a reef tank is roughly 6.5 mg/l, but that this value means relatively little to the health of your animals (see below).
| - | Salinity (ppt - 0 / 00 ) | |||||
| Temperature ( o C) | 0 | 10 | 20 | 30 | 40 | |
| 10 (50 F) | 13.0 | 12.2 | 11.4 | 10.6 | 9.8 | |
| 15 (59 F) | 10.3 | 9.7 | 9.2 | 8.6 | 8.1 | |
| 20 (68 F) | 9.4 | 8.8 | 8.4 | 7.9 | 7.4 | |
| 25 (77 F) | 8.5 | 8.0 | 7.6 | 7.2 | 6.7 | |
| 30 (86 F) | 7.8 | 7.4 | 7.0 | 6.6 | 6.2 | |
This brings up a question in my mind... do our tanks need to be at saturation? Along the surf, the water is probably saturated in the ocean, but what about 20 feet down - anybody have figures on O2 levels at various depths?
Actually, natural seawater in areas around reefs are generally between 95-110% saturated with oxygen, and >60% of areas measured are at 100% saturation Obviously oxygen is important to our animals, and such concern is justified. On the reefs of St. Croix, night time oxygen levels range from about 5-6.5 mg oxygen/l, and daytime levels rise to around 7.5-9 mg oxygen/l (saturation was roughly 6.25) depending on the reef and location. Walter Adey reports similar levels of oxygen in his ATS-based tanks at the Smithsonian, but also points out in his book that it is not the oxygen concentration itself that is important but rather the EXCHANGE rate of oxygen.
Let's say you start with an average well-stocked 50G reef aquarium. The respiration of the animals in the aquarium would likely be on the order of 3g of oxygen per hour. Even if you could supersaturate the seawater (lets say it's at 9mg oxygen/l -- the highest recorded on the natural reefs), that still only gives you about an hour for the animals to suffocate with only diffusive gas exchange at the tank surface. Of course, protein skimming, turbulent water flow (e.g., "dueling" powerheads, and especially surge devices), and photosynthesis will alter that rate of exchange, and with the normal exchange rates of roughly 4-6g of oxygen per square meter per hour, the respiratory needs of your animals should be easily met. While it is true that both salinity and temperature will affect the particular value of the oxygen saturation coefficient in seawater (e.g., if you're being good and maintaining your tank at a natural salinity of ~35 ppt -1.026 SG-, then the oxygen saturation at 25C is 6.9mg/l, while at 30C is only 6.4mg/l), this is pretty much a non-factor. If the difference of 0.5 mg/l of oxygen in your tank makes any difference to the inhabitants of your tank, you are *seriously* under-circulating or overloading your tank!
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