Don’t worry; we’re not going all scientific. There’s no need to anyway, the detail the marine aquarist needs to know is straightforward.
Oxygen is a very important dissolved gas in the seawater. The livestock need it to survive as we do – no oxygen, no life. If dissolved oxygen is in short supply then the aquarium livestock will be subject to stress, and if the oxygen level is continuously too low disease and maybe death will follow.
It isn’t just the livestock that will suffer either, the bacteria in the bio-filter will too. These bacteria operate the nitrogen cycle when ammonia is converted to nitrite which in turn is converted to nitrate. The ammonia and nitrite are toxins and will kill at quite low levels, so the bacteria’s welfare is paramount. The bacteria that convert the toxins are oxygen hungry and rely on the amount available in the seawater.
In addition if there is a good supply of dissolved oxygen in the seawater the redox potential will be reasonably high meaning a clean environment. For simplicity’s sake the redox potential is a measure of ‘cleanliness’.
Oxygen is taken into the seawater at air/water interfaces, the major one being the aquarium seawater surface with more in the sump, weirs, overflow pipes (where air is also in the pipe) etc. The intake of oxygen is dependent on efficient seawater movement, and without this movement trouble could follow.
There is about twenty times more oxygen in the air than there is in seawater. Seawater in the aquarium should have an oxygen level of between 6 and 8 ppm (parts per million). The average amount for a well designed reef aquarium is 6.5 ppm*. Not a lot really, but quite sufficient if all is operating properly. This amount of oxygen varies somewhat according to the salinity and temperature of the seawater – another reason why high temperatures bring the aquarist closer to the ‘edge’. Further, oxygen levels can fall at night when the aquarium is in darkness as, for example, algae do not photosynthesize. This can be combated by having algae in a sump which has an opposite lighting cycle, that is, the algae are lit when the main display lights are off.
All aquariums, be they coral only, fish and coral or fish only should have adequate circulation and seawater oxygen levels. However, it is the fully stocked fish only system with its higher numbers of fish that could be most at risk.
Fish place the highest demand for oxygen (I do not know the demand for oxygen placed by bacteria) so it follows that the more fish the heavier the oxygen demand. There are two dangers – first, the demand cannot be met because the aquarium is overstocked, and second demand cannot be met because oxygen intake in insufficient.
In the modern aquarium there are some devices that assist with oxygen such as the protein skimmer. Reliance should never be placed on these devices for the purpose of oxygen supply. If they fail there could be trouble. The seawater in the sump, if one is used, should not be counted into the system net gallonage when stocking is being considered so that those gallons assist with seawater quality including oxygen.
So consider a fish only system which is fully stocked and has been stocked correctly. If the system design is good there should not be any problem with oxygen as long as everything is running. Fresh oxygen is being taken in at the air/water interfaces all the time as more seawater reaches these surfaces and is then distributed around the system.
What if a circulation powerhead breaks down? The seawater movement is clearly going to reduce, though there may still be enough oxygen intake – or there may not. In the latter case, there isn’t any adjustment by the fish to economize on oxygen usage, so they will exist on the oxygen that is available. The demand will reduce the oxygen until it has reached critical levels, and the danger of suffocation arises. This of course will be made worse by the demands of the bacteria in the bio-filter. The same applies with a power cut when all circulation ceases. The reduction in the available oxygen could be more rapid and the danger of suffocation would arise more quickly.
It is clear that stocking is an area where great care needs to be taken. In the reef aquarium there is a smaller danger of oxygen problems as seawater quality is protected by having less fish though care still needs to be taken. In a fish only system with its heavier fish load, and not forgetting to consider the higher numbers of oxygen hungry bacteria there will be, the danger of oxygen depletion is higher.
The guidelines for stocking both reef and fish only systems are readily available and should not be exceeded. In addition, for peace of mind and especially in areas where power cuts are known to occur, the aquarist may wish to consider back-up battery operated powerheads, or even a small back-up generator with enough power to drive the aquarium circulation system and heaters.
No aquarist would wish to see the expensively furnished aquarium suffer or even die because of inconsiderate stocking or the event of power loss. The life in the aquarium deserves better than that.