Part 5: Dissolved Oxygen

The Stuff That Water’s Made Of
Part 5: Dissolved Oxygen

by Lenny Llambi
First published in Fincinnati, the official newsletter of the Greater Cincinnati Aquarium Society
Aquarticles.com

Up until this final article, we have covered a variety of topics within the realm of water chemistry. Topics like the nitrogen cycle are very common within aquarium literature; whereas other topics, like conductivity, are rarely discussed in any detail. I think its safe to say that this month’s topic is one of the least discussed water parameters: Dissolved Oxygen (DO). Its actually no surprise that DO is not a popular topic of discussion in aquarium literature. Life-sustaining levels of oxygen are easily maintained in the aquarium without much extra effort or thought. However, it seems like every time I have a catastrophic, mass extinction in an aquarium it is due to overlooking the aquarium’s oxygen requirements.

Oxygen is important to so many living creatures; including: plants, bacteria, algae, fish, and invertebrates. It is used to break down sugars all the way down to carbon dioxide and water, thus releasing the most amount of energy possible from the sugar molecule. When an organism suffocates; the lack of oxygen literally brings every single bodily process that requires energy to a grinding halt. We terrestrial creatures are blessed with an atmosphere that consists of about 20% oxygen. Your tank is a text book model of a well-oxygenated aquarium, if the water column can hold on to half that much. In order to attain “text-book” status, you will need to maximize the amount of oxygen being dissolved into water; while minimizing the amount of oxygen being stripped out of the water column.

Preparing the Water
You may want to refer back to the first article in this series, because I explain how gases interact with water, a little more in depth. It is important to remember that water cannot dissolve an infinite amount of chemicals. Therefore, the more chemicals that a particular body of water dissolves, the “less space” it has to dissolve any extra chemicals. Since saltwater dissolves so many more salts and minerals than freshwater, DO levels in marine environments are about half that of freshwater. Temperature also affects water’s ability to dissolve gaseous chemicals. Colder water dissolves gases more readily than warmer water. This is evident in small farm ponds where the stock-fish can be found gasping for air on the hottest summer day. In addition to all of the toxic chemicals that industrial facilities can pollute waterways with, many industrial facilities raise the average temperature of adjacent water ways. This is known as thermal pollution. The rise in temperature decreases the amount of DO in the water, causing mass-suffocation, which in turn raises dissolved, nitrogenous wastes and lowers pH, as the dead fauna decomposes. This is in effect what happens in many aquarium crashes. When DO Is used up faster than it is replenished, animals begin to die by suffocation. The decomposing bodies increase nitrogenous waste concentration and lower pH (see part 4) as the cadavers are decomposed, further stressing and poisoning fish. Essentially every thing that could go wrong goes wrong when DO becomes deficient.

Consuming Oxygen
The most obvious consumers of oxygen in our aquariums are fish. As body mass and activity increase, a fish’s need for oxygen increases as well. Even though a kribensis (Pelvicachromis pulcher) is larger than a rummy-nose tetra (Hemigrammus rhodostomus), the rummy-nose tetra compensates with its constant activity, which requires quite a bit of oxygen to fuel its metabolism. Moreover, higher temperatures not only hinder water’s ability to dissolve oxygen, they also cause fish’s metabolism to rise. When metabolism increases, the body needs to consume more oxygen in order to burn more sugars and create more energy. Temperature spikes are a double-edged sword for this reason. However, this works in reverse as well. The nest time you come home from an auction with one too many fish (they were rare and seldom-seen in the hobby after all), and you have to keep more fish in one tank than you know you should keep together: turn the temperature down. This will lower the metabolism of your fish, thus lowering their need for oxygen, not to mention the cooler water is now able to dissolve more oxygen.

Of course, any invertebrates that you may keep also use oxygen, but it is the oxygen consumption of an unseen inhabitant that makes up the next largest oxygen consumer in an aquarium. These unseen inhabitants are the nitrifying bacteria that are involved in the nitrogen cycle. The entire process of decomposing a piece of food down to nitrogen gas, carbon dioxide, water, etc. uses oxygen every step of the way. If there is a large amount of decaying, organic matter in the aquarium, DO will naturally fall as the increased number of bacteria consume much more DO. This is yet another complication in the thermal pollution scenario. As larger species suffocate, and begin to decompose, the increased bacterial population consumes even more of the precious little oxygen left. The fact that these invisible bacteria can consume all of the DO in your aquarium is disconcerting, but it underlines the importance of watching your aquarium. When fish begin to suffocate, they become lethargic and discolored and gasp for air at the water’s surface. Snails are an excellent indicator of DO, as the snails will collect at the water’s surface when DO decreases. The possible causes of a drop in DO are numerous, so it really is better to just keep a close eye on your fish’s behavior.

Dissolving Oxygen
The first way that oxygen makes its way into water is from the atmosphere. Anywhere that water and air interface, gases move from the substance with a high concentration of gas to the substance with a low concentration of gas. In the case of oxygen, it diffuses from air (higher concentration) to water (lower concentration). There are several ways that we can manipulate this fact so that DO is maximized throughout our aquarium. The most obvious place that oxygen will diffuse into water from air is the water surface. Therefore, you must leave some room between the water surface and your hood so that the water is exposed to air. Obvious as that point may seem, your hood is not the only thing that can get in the way of atmospheric oxygen diffusing into your water. I have recently moved into an old carriage house that I am slowly renovating. Needless to say, there is an endless supply of dust. One day, a hang-on filter broke on one of my aquariums (of course in the middle of some major sanding), which caused a thick layer of dust to collect on the top of the water. I did not notice anything until I walked by the aquarium and found all of the fish nearly dead, gasping at the top of the tank. I quickly found that the filter had ceased working, and within minutes of replacing it with an operational filter, the dust layer broke and all of the fish were happily swimming about as if nothing ever happened.

Just realize that its not so much the movement of water that prevents the dust layer, it is the “breaking of the water surface”. This is called water agitation, and also helps to increase the amount of DO in the water. Some of the most highly oxygenated bodies of water are fast flowing streams, with lots of white water. As a matter of fact that “white water” is just water with lots of teeny-tiny air bubbles produced by the fast flowing waters churning over rocks and down steep drops in elevation. The most obvious ways to reproduce this is to either use an air pump blowing through a diffuser, or to use venturi injection, which is now an option with almost all major brands of powerhead. Saltwater enthusiasts, gain increased DO as a side benefit of using a protein skimmer. The skimmer is primarily used to remove harmful toxins in the saltwater aquarium, but the large amount of micro, air bubbles used to accomplish this goal, also increases DO. If you don’t like the look of air bubbles in your aquarium, even a hang-on type power filter, where water cascades down into the tank, breaks the water surface, and produces air bubbles below the surface will increase DO.

There is one final point to take into account. If you were to take a 5 gallon bucket filled with only water and measure the dissolved oxygen throughout the water column, you would find that the water near the surface of the water has much more DO than the water at the bottom of the bucket. In deep, placid lakes where there is not a lot of water movement, the lower portion of the lake often times has insufficient oxygen to sustain significant populations of fish. So this means that we need to make sure that the water column in our aquaria is thoroughly mixed, or ‘turned-over’.

The second way to get oxygen into your water is through the use of live plants. Plants produce oxygen as one of the final by-products of photosynthesis. The famous aquascape aquarist , Takashi Amano, actually recommends that anytime you add fish to an aquarium, you should also add some sort of plant life to not only add DO, but also to help with filtration. Now don’t go thinking I’m telling you to go out and buy expensive light fixtures for all of your tanks. There are a myriad of plant species which will grow under a good ole fashioned strip light. Java fern, Java moss, all Anubias sp., most Cryptocoryne sp., hornwort, and Najas Grass are all good plants that can be used under a normal output light. Actually, well-known fish importer, Tony Orso, grows all of his Anubias sp. using the lights on his ceiling (yes you read that right). Moreover, with the exception of Cryptocoryne sp. you can grow all of these plants without a substrate. I’d say most people stay away from live plants, because of the extra effort, but as long as you stay on top of your water changes, the choices that I provided above will help maintain and even increase DO in your aquarium.

I hope that all of these painfully technical articles have broadened your understanding of water chemistry in the aquarium. A lot of people think of water chemistry as a laundry list of numbers that your water test kits needs to conform to whenever you test your aquarium. These people probably think that I test my aquariums weekly with the finest water tests. With the exception of salinity in my saltwater tank, I can’t remember the last time I opened my “el-cheapo” brand water test kit. I rely on the greatest test kit of all, my eyes. You see, I know what my fish and invertebrates are supposed to act like and what my plants and corals are supposed to look like, because I observe my tanks every single day, even if it is just a minute. Whenever any of my aquarium’s inhabitants appear abnormal, I know something is wrong. From this point, I think back and try to identify any maintenance that I may have neglected to perform. Usually I realize that I have not changed the water in a while. Next, I try to identify if I have performed any maintenance differently. Sometimes I realize that I dosed a different amount of a chemical, or that I did not adjust the pH and conductivity in an acidic, soft-water tank. If I still cannot identify the problem, then, and only then, do I blow the dust off of ole reliable, my water test kit and test every water parameter. Perhaps this is all overkill for the average aquarist that maintains a couple of show tanks, but for those of us that could justify charging admission to our basements, understanding a basic level of water chemistry makes caring for all of our many fish less problematic. In the meantime, let’s keep learning about and caring for our fish.