syngnathid.org - Oxygen Saturation

Oxygen Saturation

Oxygen saturation is somewhat obscure test for almost all but the advanced hobbyist. Typically, this water parameter is only in question in very heavily stocked tanks, or tanks where gas exchange is impaired. However, with the introduction of captive-bred, deep-water seahorses such as H. abdominalis and H. ingens, it has become more of a concern for seahorse keepers. The reason for this is that these fish live in cooler waters, where there is more available oxygen saturated in the water column. This leads us to our first point:

As Temperature Rises, Available Oxygen Diminishes

Dissolved oxygen in water is a function of the specific gravity and the temperature of the water. So, as the temperature of a body of water (or the specific gravity) increases, the oxygen available in the water decreases. This is due to the fact that oxygen is a highly reactive gas... What this means to your fish is that they will experience asthma-like symptoms such as labored breathing and listlessness. In the short-term, this condition is an incredible stressor for the fish, which will leave them open to opportunistic infections. In the long-term, the fish will suffocate and die. If you plan on keeping deep-water or temperate-water fish, be sure you know their native habitat's temperature and be sure you can provide this for them. So, the moral here is, know the needs of the fish you want before you buy them, and be sure you can provide for these needs.

Increasing Oxygen Content

There are two simple ways to increase the amount of dissolved oxygen in the water column. First is to lower the temperature. This can be done by using a chiller, floating frozen bottles of water in the tank, or by the use of air currents over the water's surface. This last method provides and additional benefit which is also the second method of increasing oxygen: enhance gas exchange. By placing a fan blowing almost parallel to the water's surface, you will greatly enhance the exchange of CO₂ and O₂ in the water. The more CO₂ you get out, the more O₂ can fit in. This is especially helpful in densely stocked tanks where the respiration of the organisms in the tank are generating copious amounts of CO₂. Also, you can enhance the effectiveness of the fan by have a powerhead blowing just below the water's surface so that it ripples the water gently. This helps to break the surface tension and facilitates the removal of the CO₂ molecules, as well as the absorption of the O₂ molecules.

Table 1 below gives the amount of dissolved oxygen in parts per million, representing 100% saturation for various specific gravities and temperatures. To calculate your percentage of dissolved oxygen, test the amount of oxygen present in your tank (in parts per million), and divide the number from Table 1 corresponding to your tanks parameters into your result. The answer is the percentage of dissolved oxygen in your system. Answers over 100% are perfectly valid, and indicate super-saturation of the water.

For example, let's take a tank with the following measurements:

Specific Gravity = 1.026
Temperature = 79°F
Dissolved Oxygen = 4.9ppm
The amount of dissolved oxygen at 100% for this tank (from Table 1) is approximately 6.9ppm.
Then we divide: 4.9ppm measured dissolved oxygen ÷ 6.9ppm dissolved oxygen at 100% saturation = 71% saturation.

This example tank, is on the border of being in trouble. Using the methods described above, the keeper of this tank would probably notice decreased respiration in the fish, more activity of all organisms, more color, and overall better health and robustness.

Table 1

Temperature Specific Gravity @ 25°C

0
(Freshwater) 20
(1.015) 25
(1.018) 30
(1.022) 35
(1.026)

10°C / 50.0°F 11.3 9.9 9.6 9.3 9.0

20°C / 68.0°F 9.1 8.1 7.8 7.6 7.4

22°C / 71.6°F 8.7 7.8 7.6 7.3 7.1

24°C / 75.2°F 8.4 7.5 7.3 7.1 6.9

25°C / 77.0°F 8.3 7.4 7.2 7.0 6.8

26°C / 78.8°F 8.1 7.2 7.0 6.8 6.7

28°C / 82.4°F 7.8 7.0 6.8 6.6 6.4

30°C / 86°F 7.6 6.8 6.6 6.4 6.2