If you’re like most marine aquarium hobbyists, you probably don’t give a great deal of thought to the levels of dissolved gases in your system beyond making sure you’re providing good aeration and circulation. But under certain circumstances, these levels can get out of whack to the detriment of marine livestock.
All aquarists are aware that their aquariums require a minimum concentration of dissolved oxygen in the water for their fish to survive. Many aquarists are also aware that if gases are dissolved in too great an amount, supersaturation can occur, causing serious health problems in their animals. Due to a lack of appropriate test equipment, most home aquarists can do little about measuring for potential problems with dissolved gas levels.
There are four basic concerns regarding the level of dissolved gases in aquarium water: acute supersaturation, chronic supersaturation, low dissolved oxygen tension, and high carbon dioxide tension. Even without test kits and expensive meters, aquarists can avoid problems with these concerns if they are made aware of the issues and take certain precautions.
This is known as the “gas bubble disease.” An aquarium that has a water pump malfunction of some sort may develop a dissolved gas saturation level of greater than 120%. The onset is sudden and the results devastating. Fish will develop severe bilateral exophthalmia (pop-eye involving both eyes), and their gills will show massive trauma with aneurysms. In the worst cases, air bubbles will be present in the soft fin rays and in the gills. Death is rapid, and even if the still-living fish are moved to a new aquarium, they will usually not recover.
Two common causes of acute supersaturation are a sump that is allowed to run dry, allowing the pump to suck in air, or a loose fitting that allows for a continuous air leak at the suction side of a pump. Despite best intentions, equipment sometimes does fail, allowing supersaturation to occur.
In these cases, the problem can be lessened by always having the return line from any aquarium pump return water to the aquarium above the water line. The agitation of the pumped water hitting the aquarium surface is akin to shaking up a can of soda pop and driving off the “fizz.”
Not every case of gas buildup in animal tissue is caused by supersaturation. Some bacteria produce gasses as they grow and multiply. A fish with a serious bacterial infection may develop gas pockets in the intestines, behind the eyes, in the air bladder, or even under the skin.
Only a few aquarists have ever identified this relatively rare, bizarre syndrome. Essentially, this disorder occurs when the level of supersaturation is not great enough to kill the fish outright, but sufficient to cause physical damage to the fish. It is a great mimic of other problems. For example, the damage caused by the supersaturation may be invisible to the aquarist, but it weakens the fish. In turn, the fish may develop protozoan infections or may die from a pre-existing chronic problem, such as fatty liver disease.
Since mechanical failure of a pump rarely occurs in a “partial fashion” (that is to say, only a tiny leak develops), this problem is rarely the cause of chronic supersaturation. The primary cause of this malady is the use of very cold tap water (even if subsequently warmed up) to perform partial water changes. During the winter at northern latitudes, the temperature of the tap water may fall to 38 degrees Fahrenheit. At this low temperature, water can hold a huge amount of dissolved gas. In addition, the city water supply may use powerful pumps that force even more gas into solution. When this water is used by unsuspecting aquarists, the result is chronic supersaturation (or even acute supersaturation in severe cases).
One answer to this problem is to warm up the tap water to 85 degrees Fahrenheit (30 degrees Celsius) and aerate it heavily for 48 hours prior to using it for a water change in an aquarium.
The following are symptoms of chronic supersaturation in fishes:
- Mostly smaller fish are affected (they have a higher surface-to-volume ratio).
- White “mask” on nape of head.
- Mild exophthalmia, sometimes only involving one eye.
- Discoloration of posterior soft fin rays.
- Missing scales, fin damage (resulting from aggression from the less affected fish).
- Fish loss attributed to other chronic problems, such as fatty liver disease.
- Gill aneurysms, some macroscopic air bubbles may be seen.
- Rapid, deep breathing.
- Listlessness, hanging near the surface or on the bottom.
Generally, these symptoms abate only gradually once the source of the supersaturation has been eliminated. In some instances, fish mortality continues for weeks after the incident has been resolved.
Low dissolved oxygen tension
This is the relatively common “gasping fish syndrome,” in which the dissolved oxygen level of the water drops below that required by the fish. The fish will be seen breathing rapidly and deeply, often gasping at the surface. As the problem progresses, the fish may die, often with an unmistakable “last-gasp”—their mouth being fixed wide open.
Common causes of this problem include overcrowding, insufficient aeration, chemical removal of oxygen (such as by potassium permanganate), and low saturation levels due to high water temperature and high biological oxygen demand from microorganisms in the aquarium.
Generally, if the affected fish are moved to a new system that has sufficient levels of dissolved oxygen (or if the problem is quickly corrected in the original aquarium), the fish will recover spontaneously with no long-term effects. Be aware, however, that bacterial, protozoan, and metazoan gill diseases can cause similar symptoms.
High carbon dioxide tension
In this rather rare instance, the dissolved oxygen level may be at or near saturation, but artificially elevated carbon dioxide levels create symptoms in the fish that mirror those of oxygen deficiency. There is only one situation in which a marine aquarist is likely to see this problem: in a heavily stocked aquarium that has powerheads or canister filters whose effluents do not actively break the surface tension of the water. Breaking the surface tension allows the excess carbon dioxide to be driven off.