12-11-2011, 12:29 AM
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Air pumps actually add very little oxygen to the water. Their main function is water movement. Other then that I see no point to them. I guess they can provide aesthetics if you like the look of those bubbles, I personally don't though. I have 2 air pumps but they rarely get used. The bubbles produced by an air stone are simply way to big to add much of any oxygen to the water. They quickly float to the surface. If you actually wanted to add oxygen with bubbles you would need to diffuse it like you do with CO2. I use ceramic bubble diffusers for CO2 and can easily raise it 6 times higher then normal levels. The only difference is the size of the bubbles. They are extremely small and it takes them a long time to reach the surface for a bubble. The smallest ones may never make it to the surface before diffusing into the water. This size is due to the diffuser and its pore size. You can't even see the pores on a ceramic CO2 diffuser and it takes a lot of pressure to push the CO2 through it. Thats why we call it pressurized CO2. DIY yeast CO2 normally doesn't work with the larger diffusers I have because its hard to build up enough pressure to produce those small bubbles. In the end it doesn't matter how much CO2 or O2 you pump into the tank, what matters is how well you diffuse it. Smaller bubbles=more surface area=better diffusion.
Very few tanks ever have oxygen problems. Oxygen is introduced by diffusion and surface movement and agitation can help increase this. I like little to no surface movement in all my tanks, definitely no agitation as its a waste of CO2. Rarely do I have fish hanging at the surface. Even my youngest soil tank that has pretty anaerobic substrate and high fish load can sit stagnant for over a day before the fish start sticking to the surface.
A tank with a lot of productive(growing) plants and minimal surface movement can get supersaturated with O2 for part of the day. Plants produce O2 and a lot of it diffuses into the water. Sometimes you get what is called pearling, where oxygen bubbles on the plants then floats to the surface. One reason this happens is the plants are producing oxygen so fast that it has caused the water to become supersaturated. When supersaturated the diffusion rate into water drops and the result is build up of oxygen on leaves until the bubbles are large enough to break free and float to the surface. When production rates drop so does the oxygen level, also considering that plants consume oxygen at night.