O2 is only one of the gasses we deal with in water (H2O). There are a multitude of dissolved nutrients, chemicals, solids and gases. Individually they are important but it is their combined interactions that tell us if the water in the system is ideal for crayfish or any aquatic life at all.

Controlling the dissolved gasses is one of the most important parts of water treatment and quality. The four principal gases dissolved in water are, Oxygen (O2), Carbon Dioxide (CO2), Nitrogen (N2) and Argon (Ar).

A balance is maintained of all bulk gasses, when air is in contact with water, this is called saturation. Changing that balance between air and water results in either super-saturation and under-saturation of any of the given gases. The critical controlling factors of dissolved gases in water are temperature, pressure and salinity.

The physiology of the crayfish and fish we introduce to the water also produces super or under-saturation. If a species consumes O2 then under-saturation will occur and produces CO2 creating a supersaturated environment of CO2.

The ideal environment in aquatics is saturation, being the balance between air (not O2 but the air we breath, air only contains on average 21% O2) and water. Or super-saturation of the primary gas the species needs, in this case oxygen and for plants this would be Co2

The key question now is; How do we achieve a saturated environment that is constantly changing with the addition of feed and the species growth? Taking into consideration the larger the species grows, more feed is required adding to suspended solids, more O2 is needed (BOD we will talk about in another issue) and more CO2 is produced?

First we need to deal with the bulk gases we dont want (nitrogen and argon) and then we need to look at species requirement and tolerance.

To do this we will rely on the basic principals of gas transfer. This theory relates to the process where the gaseous form of a compound is dissolved into or driven out of the water. Firstly the gas moves to the gas-liquid interface where the gas molecules diffuse through the laminar gas and laminar liquid films. After this the gas is completely disolved and becomes part of the water or liquid.

The rate at which gas is transferred from air to water or out of water is proportional to the area of the gas-liquid interface and the difference between saturation concentration and the actual concentration in the water. This dictates the direction and rate of gas tr4ansfer at the gas-water interface.

Quote:

Henry's law determines the maximum solubility of a gas under a given set of physical conditions, and this is the saturation level. The solubility of a gas depends on the temperature, salinity, atmospheric pressure and the mole fraction of the gas. Reference Material Download

From Henry's Law we can determin the rate of dissolution of a gas into water is proportional to:

The difference between actual and saturation concentrations of the gas in solution
The area of the gas-liquid interface
The thickness of the liquid film
The volume of water into which the gas is entering
The diffusion coefficient.

So, in the end of all that I believe that you can have too much Oxygen in your water which will create an imbalance of the other gasses which in turn will mess up the chemistry.

For example; The more Oxygen (o2) you have in your fish tank the less room for Carbon dioxide (Co2). Which means that the Co2 will gass off or be "pushed" out of the water and the result will be your pH. will rise. Also this type of environment is not ideal for your favourite aquatic plants.

The opposite also applies in general without quoting the exact sciences. I may do another article regarding the relationship between Co2 in your tank and the pH. next week.

So stay turned!