The marine aquarist does all sorts of water parameter checks to ensure the quality is high. A lot of money is spent on equipment and livestock. Once the livestock point is reached, occasionally problems will arise, particularly with fish in a fish only system. Though the aquarist may well be close to a nervous breakdown as all the water checks done do not cause any or much concern, the problem could be to do with oxygen.
Why should a fish only system be more likely to have a problem than a captive reef? In a captive reef it is usual to have less fish inches relative to seawater net gallonage than in a fish only system. Fish demand oxygen as do all living organisms (ok, there are some that derive their oxygen in less usual ways, but as far as I know everything needs oxygen). A heavy fish load will clearly demand a lot of oxygen.
The problem is rare. However, it does pop up now and again, usually with novice aquarists, and it is not that much a rarity. It often starts in the summer.
The aquarist, as said, has a heavy fish load, but is not concerned as they all seem fine. The bio-filter copes. Then along comes some warm weather. As would be expected, as the seawater heats up because of the increased temperature the heaters turn off. The temperature increase continues. Lighting is in use, and it is this that is heating the water, particularly metal halides. The aquarist does not have a chiller (seawater cooler) as they are expensive and the occasions the device could be needed are considered insufficient with regard to the cost.
The seawater contains 6 to 8 parts per million (ppm) oxygen in a well managed average tropical marine aquarium. Not a lot really. Warm seawater contains less than cold and this is a tropical aquarium.
It should be remembered that it is not only the fish that are using oxygen. There is the bio-filter. Unless this is a trickle filter, all the oxygen for the oxygen hungry bacteria is taken from the water. Oxygen supply could be getting near the edge.
If there is a sump with macro algae in it, then during daylight hours this will assist in oxygen replenishment. It has limits though. A properly functioning protein skimmer will also assist. Nevertheless, the situation is not as it should be.
Fish breathing rapidly, possibly combined with hanging around near the water surface, is a sign of oxygen depletion. So why has this happened?
First, it may be that the aquarium is overstocked with fish. This should never occur despite the temptations, as there is obviously an increased demand for oxygen, and the bio-filter has to work that much harder employing more bacteria – and as said these bacteria are oxygen hungry. Then the water temperature increased. This can happen to the best designed and cared for aquarium, and the first thing to do when a significant temperature rise is noted is to switch off the lighting, particularly metal halide. There are further ways of cooling the seawater that will not be gone into here.
Though problems can arise in any system, it is usually the aquarium that has been recently equipped and stocked by a novice that are struck. That’s the clue.
In a well designed marine aquarium system of any type consideration has been carefully given to water circulation to ensure maximum gas exchange, among other things. Water can be circulated in many ways but a commonly used example is the powerhead. Whatever device(s) are used for circulation, it is essential that the seawater moves sufficiently so that efficient air/water interface(s) occur. The usual major interface is the water surface, and it follows that if movement is adequate then the water will continually reach the surface before moving on and being replaced with more water – a continuous gas exchange. In addition, aquariums with sumps often use weirs over which the water flows, creating another efficient gas exchange area. The sump water surface will provide yet more gas exchange.
It follows that the system with the problem will often be found to have sluggish water movement. Therefore gas exchange is minimised. It may have been an attempt to economise with equipment, and if so this is not understandable, as the cost of say powerheads is low compared to other items and livestock. Definitely a false economy.
In a well designed system, be it ready built or aquarist built, there is no need to obtain oxygen test kits for regular testing, unless the aquarist has an interest in what the oxygen content of the particular seawater is. As said, it would be unusual for an oxygen problem to arise, although when water temperature rises above the design point a watch must be kept for any sign of distress, particularly in a fully stocked fish only aquarium.