Here's a series of articles on the subject:
The Need to Breathe in Reef Tanks: Is it a Given Right?
The Need to Breathe, Part 2: Experimental Tanks
The Need to Breathe, Part 3: Real Tanks and Real Importance
Perhaps most interesting were the results of illuminating small tanks containing a handful of macroalgae. In the previous article, the fifteen-gallon tank described above used the same clump of Chaetomorpha that was described in this article. In the fifteen-gallon tank made hypoxic through the use of nitrogen and illuminated by a single 15-watt fluorescent lamp, little oxygen was provided to the tank by the algae. In the ten-gallon tank described in this article, the irradiance was provided by a 65-watt lamp. So long as water flow was provided, the tank increased its oxygen content only minimally, and it is hard to say if the same increase would have occurred without the algae. Once the water flow was turned off, however, oxygen levels rose quickly. I believe this occurred because the water flow caused much of the oxygen produced by the algae to be lost at the air/water interface. With no other organisms present (besides, obviously, any microbes present), oxygen levels remained supersaturated in the tank over many hours; longer, in fact, than I would have expected given gas exchange at the surface. The reduced effect seen previously in the fifteen-gallon tank probably resulted from irradiance levels that were inadequate to maximally stimulate photosynthesis. This experiment shows algae's potential under sufficient irradiance and slow flow, such as the conditions found in refugia, to effectively raise oxygen levels in tank water.
Edited by seabass, 20 March 2012 - 10:32 AM.