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.
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