Should I keep the air pump of my fish tank on all the time? What is the rate of oxygen absorption in water at 25 degrees Celsius? I have a primitive type of aeration system where a thin tube is suspended in water with its other end connected to the air pump.
Yes, you need to keep the airpump running 24/7. It is the most important to keep the pump running at night to keep the water oxygenated when your plants are not producing oxygen.
During the day your aquarium plants will produce more oxygen than your tank uses so the oxygen saturation will stay at 100%.
As long as you keep the water moving your water will be close to the saturation point.
The important thing with keeping the water moving is to get rid the CO2 that builds up in the water from your fish' respiration. It is not the lack of oxygen that might kill your fish, but it is the build up of CO2 that blocks the uptake of oxygen in your fish.
The amount of dissolved gasses water can hold is largest at low temperature and falls rapidly with increasing temperature (I do not have the numbers but at 25+ °C you need to keep the water moving to increase the gas exchange to keep your fish at their optimum health).