One of the basic principles of good aquarium maintenance is to make sure that the
water is well aerated, so that enough dissolved oxygen is available for fish respiration.
This oxygen can be produced by the photosynthesis of aquatic plants and algae, or it
can simply diffuse from air to water. In most environments it is diffusion from air
that represents the main source of dissolved oxygen. That’s certainly the case for
most aquaria. (In that respect, one should know that filtration systems are useful not
only for cleaning the water but also for stirring it, bringing bottom water to the
surface where it can take up oxygen from the air before being pushed back to lower
levels where the fish can enjoy it. Air bubblers perform the same function and, as an
additional benefit, they increase the area of the air-water interface.)