Back to DO:
My own DO measurements support a lot of what was discussed here in this article:
The Need to Breathe, Part 3: Real Tanks and Real Importance
It's a long article but I'll just briefly post a little bit of it here.
http://reefkeeping.com/issues/2005-08/eb/index.php
Tank 1: "Clownfish Tank"
Test 1: Circulated with powerheads and no skimmer: Morning DO saturation: 16%
Test 2: Aerated with protein skimmer: Lowest DO saturation: 81.2%
Same tank but aerated with airstone: Brought DO saturation up from 51% to 87% in 30 mins.
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Tank 1 Discussion
The ten-gallon tank containing clownfish has been set up as an unskimmed system with what I consider to be an average stocking density of organisms for a tank of its size. I had assumed (wrongfully) that oxygen was maintained at high levels through the use of two powerheads that agitated the water's surface. However, once the lights went out and photosynthesis stopped, oxygen levels dropped quickly from a high of 78.7% of saturation to a hypoxic low of 16% of saturation. The levels were apparently low enough that each night, the clownfish would leave their anemone and adopt a position just under the water's surface directly above a powerhead. Out of concern, I then monitored the changes in oxygen levels at night using an airstone. Oxygen rose quickly and dramatically. At that point, I added a skimmer to the tank, with the result that oxygen is now maintained at much higher levels, ranging from a high of 130% of saturation to a low of 81.2% of saturation. However, it is only when the lights come on that oxygen reaches saturation or becomes supersaturated. It is notable that there appears to be a period early in the day when oxygen levels are maximal, with a depression to slightly subsaturated levels over the course of the afternoon. Also notable is a slight, but noticeable, drop in oxygen immediately after feeding. This measurement has been made repeatedly and is consistent.
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"-Aquaria can and do become hypoxic at night and such a state may pose a risk to hypoxia-intolerant organisms.
-Aquaria can and do become saturated or supersaturated with
oxygen during the day, and this is a result of
oxygen resulting from irradiance of photosynthetic organisms. In no case was saturation or supersaturation measured without photosynthesis.
-Airstones and skimmers appear to be a very effective means of oxygenating small water volumes.
-Powerheads and recirculating pumps do not appear to greatly increase the
oxygen saturation state of seawater aquaria.
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