Oxygen in the water comes mostly from the souranding air (gas diffusion process) and from the photosynthesis processes. Photosynthesis will occur only if there are plants in the aquarium and there is a source of the light. At night when there is no light plants will start absorbing oxygen from the water. The solubility of oxygen in the water depends on several parameters like:
ratio of the total weight of the water in the tank to the surface size of the water - the larger the surface of the water, the easier the dissolution of oxygen in water
how the surface water is agitated - ripples and the dynamic flow of water (waves, waterfalls). This increases the effectiveness of water oxygenation which in turn increases oxygen saturation of the water in all its points in compared with static tanks.
depth of the tank - in deeper tanks there is less oxygen at the bottom due to it's usage by organic elements within the substrate,
time needed to achieve a balance between the concentrations of the oxygen in the water and the atmosphere
temperature of the water - with an increase in temperature the solubility of oxygen decreases in the water. Also higher temperatures increase the rate of oxygen being consumed in the biochemical processes.
atmospheric pressure - if atmospheric pressure drops then the solubility of the oxygen in water reduces.
intensity of photosynthesis -this process is closely related to light intensity, the amount of the carbon dioxide dissolved in water and water temperature. Under favorable conditions, the concentration of dissolved oxygen in the water can increase by about 1 mg/litre)
salinity(saltness) of the tank - if the water in aquarium will increase in salinity than the solubility of oxygen decreases
substances like oil or a protein membrane which occur in the surface of water will effectively inhibit the exchange of gases between the water and air.
If the dissolved oxygen in the water is in equilibrium with the oxygen present in the atmosphere then it means that water is one hundred percent saturated with oxygen. Also keep in mind that the air-water system always trying to get to balance of the oxygen content.
We say that there is the lack of oxygen if there is a difference between the concentration of the oxygen under the given conditions (at the same temperature and pressure), and the actual amount of oxygen in the water.
The water is supersaturated with oxygen when the concentration of dissolved oxygen in the water is greater than in the atmosphere (at the same temperature and pressure). This phenomenon is conducive to a rapid rise in temperature or sudden pressure drop and increased photosynthetic processes. It may cause a fish disease called gas or bubble disease - water saturation of 30% will kill all your fish in the aquarium.
The effects of insufficient amount of the oxygen in the aquarium are:
diseases of fish and other organisms
bloom of algae (mainly cyanobacteria)
dying plants due to the increased in amount of the macro-and microelements in the water
intensification of the processes of denitrification, ammonification (due to the creation of anaerobic conditions in dead organic matter residual in the bottom)
We use additional aeration in the aquarium when we observe that our fish are spending most of the time just below water level and we already eliminated other possible causes of this condition. Also if we have to many fish comparing to amount of plants in the aquarium.
Aeration of the aquarium is an artificial induced process, which is increasing aeration of the water and to equal its concentration at all points of the aquarium. Additionally oxygen contributes to the remove unwanted gases from the water (for example, carbon monoxide) and is involved in the partial oxidation of the pollutants.
Water aeration efficiency is affected by:
water temperature - solubility of the oxygen in water decreases with increase of the temperature
air bubble size - the bigger the bubbles are then the amount of oxygen dissolved in water reduces
flow of the water - forces a longer way for a bubble and equalizes oxygen concentration at each point of the tank
bubble path length - if this path is longer then more time the bubble has to release oxygen to the water
permeability of the surface layer - if surface is blocked (eg, the occurrence of membrane protein or oil), the diffusion of oxygen is significantly reduced
In aquarium additional aeration is done by:
additional plants (natural aeration)
aquarium sprinkler (used mainly for surface agitation)
diaphragm pump - pump pumping air through the membrane
piston pump - air pumped by moving a piston in a cylinder
turbine aeration - air is pumped through the rotor
ankle or aeration curtain - additional elements which bring a stream of air which breaks the water surface and increase the water aeration.
When selecting the aeration equipement we should pay attention to the following parameters:
performance [litres of air / h]
maximum lifting height [m]
maximum aquarium capacity that equipement can handle [l]
number of air outlets
number of membranes