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Oxygen In The Aquarium

by Martin Thoene last modified Oct 27, 2007 11:31 AM

By Martin Thoene.


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Oxygen In The Aquarium - Why it is so important?

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Life exists on this planet mainly because of the presence of oxygen (O2). Most life-forms use it to fuel the bio-mechanical engine of their bodies. How it is obtained varies from organism to organism, but generally most family groups of animals share the same method of using the abundance of oxygen in the atmosphere.

Humans use lungs, just like the birds soaring over our heads or huge whales in the oceans of the world. The fish we keep rely on gills to extract this vital element for life. Whatever form an animal may take we all share this need for oxygen.

As fish-keepers we are all aware that our fish need oxygen, but we may not be aware of just how important this requirement is and how fundamental it is to successful fish-keeping.

Back when I lived in England, I was an Emergency Medical Technician (like a Paramedic) working with the United States Air Force. I held a US qualification in the field and our training taught us the importance of oxygen. Anyone who has done even just basic First Aid training has probably heard the acronym "ABC." Like Maria said in The Sound of Music, "Start at the very beginning. It's a very good place to start."


ABC is fundamental to human patient care:

  • A - Airway - Is the patient's airway open and can they breath?

  • B - Are they in fact breathing?

  • C - Do they have circulation? i.e. is the heart working and is it doing a good job?


These 3 things are fundamental to patient care and the first things you have to assess when arriving on-scene.

If they're not happening then the patient is in big trouble.

The very structured and artificial nature of the classroom is modified in the reality of how one carries out the job in the real world. If you walk into a situation and the patient is talking to you, in an instant you've covered nearly everything above. You just need to assess the quality of breathing and circulation... You know they're happening.

Assessing respiration in a fish or fishes is more difficult. You can't put your finger on their wrist and count pulses and you can't ask them how they are feeling. With some slower-breathing species it is possible to count respirations by gill movement, but not with some fast-breathing species.


What are the problems with having  insufficient oxygen?

Bamboo cane, horizontal (no leaves)

In my article "Hillstream Loaches - The Specialists at Life In The Fast Lane," I emphasize the importance of oxygen to Hillstream loaches in particular and go into detail of exactly why that is, but all loaches have high oxygen requirements due to living in sometimes very fast-flowing waters.

Anyone who has seen Hillstream loaches in dealers' tanks will have probably formed a false impression of theseSyncrossus beauforti - Natural habitat fishes' general behaviour as few shops provide adequate conditions. They can appear dull and inactive. Those who keep them using my River-Tank manifold concept or in other high-flow, high oxygen content tanks will report that they are lively and entertaining creatures. The best way to understand why they are like this is to look at human physiology and equate what we know of our own bodies' workings to the fish. From this we can have a better understanding of the forces at work that we are unable to see.

Oxygen is the fuel for the bio-mechanical machine that is our bodies. Without it we die. With insufficient oxygen we are compromised in some way. Our bodies have various internal feed-back systems which, like sensors in a modern car, feed a computer information. This adjusts the running of the car's engine and bio-feedback adjusts things in your body too.

If you start to run, your muscles require more oxygen to work. A message goes to your autonomous nervous system and things begin to happen. Your heart-rate increases forcing more blood around the body quicker, therefore allowing the muscles to access more oxygen. This oxygen depletion requires replenishment so your breathing rate increases and usually you open your mouth because it is easier to breath deeply that way. If any one of these physiological changes doesn't happen, how fast and how far you can run becomes self-limiting. Athletes increase their bodies' efficiency at oxygen utilization by training because that and muscle strength are essential to performance.

A fish reacts to exercise or low oxygen levels in a similar fashion. Heart-rate and respiration increase. We can see the respiration rate increases if we have previously familiarized ourselves with our fish's normal mode of breathing. As with everything in fish-keeping, observation is the key. Loaches, in particular Botiidae species, will often open their mouths wider than normal to increase the capacity to draw water in and their breathing rate will be noticeably increased.

One factor that can induce such visible signs is excitement and stress. This may be caused by fighting for dominance within the group. Botiid loaches may have extremely fast respiration during such interactions. This is not a sign of poor oxygenation within the aquarium, but rather a sign of biological speeding up of heart-rate and breathing due to tension. We all share these "fight or flight" automatic biological releases of adrenaline which cause these symptoms.

Long-term rapid breathing is something that the aquarist should be really worried about as it is a sign of something wrong and can have detrimental effects on the fish. It means that the fish is having difficulty breathing. There can be several reasons for this, just as there can with humans.

  • Insufficient available oxygen
    • Not generally a problem for humans, unless confined in a small space. For fish this means an aquarium, and the aquarist is the one who determines many factors in how much oxygen is in the water.
  • Overcrowding of the aquarium
    • i.e. too many fish in too small a tank.
  • Inability to process available oxygen
    • This may be caused by physiological problems or disease.


Insufficient available Oxygen

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Oxygen dissolves in water easier if the water is moving. The exchange of gases takes place at the water to air interface, so if this is tumbling and swirling around rocks such as in a mountain stream the oxygen levels can be Sewellia lineolata, natural stream. Note extreme current.very high due to all the mixing going on. Some streams are also very shallow and this also increases the ease of thorough saturation by oxygen. A static aquarium represents a very different level of saturation because there is no water movement.

Stocking level recommendations for aquaria were developed many years ago and tended to be based on so many inches of fish per square foot of surface area or on inches per gallon of tank capacity. These recommendations were reached by experimentation in the early days of keeping ornamental fish and bear some relationship to the available oxygen exchange in such a setup. It was quickly realized that by moving the surface of the water and creating a current within the tank that gaseous exchange could be increased. Fish, like ourselves and other oxygen users, expel carbon dioxide (CO2) and this must be lost to atmosphere or used by plants (more on that later).

Such water movement was often accomplished by air pumping devices. There are certain misconceptions among many hobbyists about how these work. Little of the air in bubbles from air diffuser stones actually dissolves in the water. The rising of bubbles creates a lifting effect within the water column and this moves the less oxygen-saturated water up near the surface where it may more easily give off CO2 and take on oxygen. The agitation of the surface film by the bubbles further assists in the exchange of gases.

This lifting effect of water was utilized to a greater degree by encasing the bubbles in a tube to create an "airlift". This principle was used by various filtration designs as a means of moving water and this movement, plus surface agitation, increased the efficiency of gaseous exchange in a given aquarium.

With the advent of power-head pumps, the potential flow available increased dramatically. Well designed airliftWater surface rippling. systems can move an impressive amount of water, but to do this need vast amounts of air and correspondingly large pumps. Unfortunately, the affordable diaphragm pumps capable of such capacity tend to be noisy and the domestic locations of most fish tanks dictate a quieter method of water movement. So power-heads became the hobby norm and generally these days air-operated systems are used more in breeding setups rather than in the decorative home aquarium.

One feature of electrical powered pumps is that the highly concentrated outflow can be utilized to add air to the aquarium. By creating a restriction in the outflow pipe, a "venturi" effect is created. Most designs introduce air at this reduction in diameter and the low pressure draws air into the water flow. This seems to be more efficient as a means of getting oxygen to dissolve into the water as the bubbles produced are often smaller than those produced by most air-stones.

Power-heads were originally designed to sit on top of uplift tubes for under-gravel filters. The venturi effect works nicely when a pump is close to the water surface. In Loach tanks, particularly tanks for Hillstream loaches, we often have the power-heads mounted much lower in the aquarium. The venturi fails to operate at this lower level due to pressure differentials. In order to inject air in the same way a small air-pump must be used to force-feed the venturi. It only takes a tiny pump to produce a lot of air by this method.

Further aeration can be provided by bubble wall devices or simple air-stones. Many loach species love to play in these streams of bubbles.

So what if I don't have a lot of water movement or aeration?


Simply put, your fish will suffer.

Let's look at the human model again. Put yourself in the fish's place by equating it to the human experience of poor oxygenation.

Have you ever been in a situation where you could not breath properly? The first thing that hits you is nervousness and irritability. That is an automatic result of your brain receiving insufficient oxygen. This is not a comfortable situation for your body. Your heart rate increases. You start to panic. Can you imagine this situation continuing for days? Not a nice thought, eh? Prolonged low oxygen levels like this can lead to severe damage to vital internal organs. Your brain, heart, kidneys, liver... everything gets affected and some damage may be irreversible. Insufficient gaseous exchange may lead to carbon dioxide narcosis. This can lead to the automatic breathing triggers which monitor and regulate CO2 levels being depressed. This throws out the whole breathing system.

During shipping of fish to our countries from wherever they were sourced, poor packaging or unexpected delays can lead to extremely low oxygen levels inside the shipping bags. Some weak fish may die, but survivors may still carry permanent systemic damage that will manifest itself in premature death or further health problems. Fish with high O2 requirements like loaches are more at risk and require knowledgeable shipping methods in order to avoid fatalities or further problems.

Once they arrive in dealer's tanks and our home aquaria they still need plentiful supplies of oxygen to prosper. I habitually have masses of water movement in all my aquariums. There are also multiple filtration systems to further support the inhabitants. I feel that sufficient oxygen for the fish is so fundamental that everything else in their health and well-being hinges upon it.


Overcrowding of the aquarium

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Botia rostrata, group of youngstersIn modern aquaria, with multiple ways of mechanically increasing water-movement and gaseous exchange capability, the old formulas for calculating stocking levels are largely irrelevant. It is possible to stock at a greater density than would have been conceived many years ago. We all see masses of fish in our dealer's tanks, so it must be ok, right?

Not so. Dealers' tanks are crammed with fish for reasons of efficiency at point of sale. Good shops carry out large systemic water-changes and have the advantage of backup systems. Often the home aquarist does not have any backup system available. What if you have a long-term power outage? All the pumps in the world will not help without electricity to run them.

Stocking levels must be kept in such a way that the surface area of the aquarium still has some bearing on the number and size of fish being kept. For temporary outages it will be the sole gaseous exchange interface. Remember that inches of fish per square foot or whatever method is utilized for determining stocking level must take into account that many loaches, particularly Botiids are much bulkier than other species of similar length. All that biological mass requires oxygen. All its organs require sufficient oxygen.

Long-term power outages may leave the aquarist no option but to manually move the water-surface around or watch their fish expire. An alternative is a backup generator, but cost may be prohibitive to many or limited by other circumstance. Battery powered air pumps are also useful in power outages.


Inability to process available oxygen

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So you took on board all the above and have lots of water pumping around your aquarium and bubbles all up one end wall of the tank so everything will be ok, right?

Hopefully yes, but there can be other factors. What if the fish has a problem?

There are certain human medical conditions that can be equated to problems that your fish may experience. One of these is Chronic Obstructive Pulmonary Disease. Sufferers are invariably life-long heavy smokers... another reason you should quit! The COPD patient has lung alveola damage. These are the tiny sacs at the ends of the airways inside the lungs where gaseous exchange takes place. The damage means that patients don't get rid of CO2 very well, nor take on oxygen. Invariably, they use accessory muscles to breath rather than just regular chest expansion and their CO2 driven autonomous breathing system is so shot by CO2 overload that it ceases to function. Fortunately, the human body compensates and breathing is triggered by lack of oxygen rather than high CO2.

Now I don't know if fish work on the same triggering systems, but their lungs are their gills. But your fish don't smoke right, so what's the connection here?

The biggest problem that we see on the Loaches Online Forums is people's fish getting Ich ( Ichthyphthirius multifilis). It is the one disease that loaches are notorious for. Everyone focuses on the visible white spots on the fins and body, but often it gets forgotten that there are a lot of Ich inside the gills. These trophont phase Ich cells do a lot of damage to the gill filaments. As the gill filaments are the point of gaseous exchange for the fish, they form the same function as the damaged areas in the COPD patient's lungs. Therefore, a fish with Ich also has compromised breathing potential. This is why extra aeration is often advised while treating Ich infected fish. The problem is that just like in a COPD patient the damage caused by Ich in fish gills is permanent and depending on its severity can affect the fish to varying degrees for the rest of its life. Just as in the human with long-term reduced oxygen blood levels, it can lead to vital organ damage. Ich is far more than little white spots that go away with the application of a few chemicals.


How can I tell if my Loaches do not have enough oxygen?

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You have to be observant.

As I said near the start, get familiar with your fishes' normal behaviour.  Familiarize yourself with their normal demeanor and respiration rate.

Signs of uneasiness or increased breathing equate to the same uneasiness you experience if you feel you can't breath properly. These will tend to be the initial stages of signs with fish that don't have enough oxygen. Longer deprivation may manifest itself with other signs. Any time that bottom living fish are hanging out near the surface is a sure sign that they're experiencing oxygen deprivation. Because of this being the gaseous exchange interface it is most likely they'll find more oxygen there than anywhere else. Fish aren't stupid. Like you or I , if we feel that we can't breath in a given room, we will seek fresh air elsewhere. That is what they will do and if the situation is really bad they will actually gasp at the surface and try to suck in atmospheric air along with water.

All loaches breath via their gills, but some are gifted with accessory breathing methods. Weather loaches (Misgurnus anguillicaudatus) are one of the most commonly available species, and they are capable of breathing through their gut. They will gulp atmospheric air and expel it via the anus. A Weather loach that is habitually taking atmospheric gulps may be experiencing low levels of O2 within the aquarium and the reason should be investigated as any other residents are at risk.

Fish may be gulping at the surface for multiple reasons.

  • Insufficient oxygen saturation
    • Check for reduced pumping by any of the water movement devices in the aquarium. Plant debris or such like can block intakes.
  • A bacterial bloom
    • This will normally manifest itself as a misty "bloom" visible in the water. The bacteria themselves use up oxygen and it may overload the capacity in the tank. Check for a dead fish or uneaten food.  Check filtration systems for ingested matter that might be decaying. A water-change will be needed quickly to introduce sufficiently oxygen saturated fresh water and the cause of the bacteria must be removed.
  • Disease
    • Look for any spots or anything else unusual.


Planted aquaria.

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Planted tanks are to be encouraged. Not only are they inherently more attractive to the eye, but increase the utilization of waste products produced by the fish. Some plants will be attacked by loaches, either directly or by being uprooted during spirited searching for food in the substrate.

Some people prefer artificial plants, but they miss out on all the biological benefits of the real thing. Your loach aquarium will be better for having real plants growing within it. Plants absorb carbon dioxide and produce oxygen via photosynthesis. Most of us learn all about this in school. The problem is that this needs light to operate properly and once darkness falls, the process is reversed.

Therefore, it is highly possible for an adequately oxygenated aquarium to gradually become less well saturated and possibly become dangerously low on O2 to the point where its inhabitants are compromised. Occasionally we hear of people who switch on aeration devices only at night in planted tanks to increase the surface agitation. They do not use them during the day because they seek to prevent excessive loss of CO2 which the plants use.

For dedicated planted tanks with CO2 injection  this desire to provide the plants with sufficient CO2 and the fish with adequate oxygen may be in direct opposition, so a compromise must be found if loaches are being kept in the aquarium. Generally, a lower stocking density is advisable to give a greater margin of safety during the hours of darkness.

Keeping loaches in planted aquaria often limits the choices of usable plant species because many may end up as salad for the loaches. Loaches, with their high oxygen requirements, must be the highest priority because as you have seen in this article there is no room for half measures when it comes to providing sufficient oxygen for their muscular and energetic bodies.


It's not just the fish that need oxygen.

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While the primary focus in our aquariums is always the fish, one thing we must not forget is that there are other  users of oxygen within the aquarium.

The filtration system and every surface in the aquarium are covered in millions upon millions of minute bacteria which all help to process fish waste. Most of the exposed bacteria utilize oxygen for part of this process. There are other bacteria called anaerobic bacteria that work inside filter media without oxygen in breaking down waste products, but the vast majority of helpful bacteria need it.

Closed cannister filters or HOB type filters provide good homes for bacteria, but the highest efficiency levels for waste treatment occur in trickle type filters where the maximum exposure of bacteria to the air happens. If you do not have such a filter then it must be remembered that as well as your precious fish, the bacteria that support the ecosystem of the aquarium are also highly dependent upon adequate oxygen levels to do their job. They need it for their health just as much as the fish do.

In the event of a power outage, maintaining these bacteria is important too. It will bode well for continued filtration efficiency if they are protected. During long power outages it is often best to disconnect cannister filters from the tank system and remove the tops so that air may get to the bacteria within the filter and keep it alive.

Starting up an unopened filter after a long power outage can lead to gross pollution in the aquarium because all those beneficial bacteria are now dead and decomposing.


To close..................

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Hopefully, I have shown the multiple considerations that must be thought of when it comes to oxygen within your loach aquarium. If the tank has insufficient oxygenation it will potentially affect every element of your loaches' health and long-term well-being. The effects of reduced oxygenation will lay your fish open to disease through systemic weakness while similar fish in a well oxygenated aquarium given identical foodstuffs will prosper and be robust enough to fight any minor infections. I truly believe that sustaining high levels of oxygen within the aquarium along with excellent water quality maintained by efficient filtration and a healthy, varied diet are the fundamentals of good loach care.

Just like the patient being assessed by a paramedic. If the aquarium isn't getting enough oxygen, nothing else matters at that point. Adequate breathing is the single most important thing to all vertebrate organisms. You, me and your loaches.

Breathe well.


Martin Thoene.

Bamboo cane, horizontal (no leaves)

Picture of Syncrossus beauforti natural habitat by Anuratana Tejavej

Picture of shallow stream by Miroslav Farkac at Saigon Aquarium, Vietnam

Picture of aquarium surface agitation by Martin Thoene

Botia rostrata
group picture by Emma Turner
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Planted aquaria

Posted by Neil Upton at Sep 14, 2007 11:13 AM
Actually there is no process reversal and respiration occurs day and night. Photosynthesis results in a day-time net production of oxygen.
I grow plants and inject CO2 but I disagree that fish stocking levels need be reduced. Fish and plants compliment each other during the day time and at night time only moderate intervention is required. I aerate at night (using a simple timer switch) and by locating an air stone in close proximity to the CO2 bubbler (DIY pop bottle) I expel as much CO2 as possible when levels are elevated due to respiration.
Under optimum growing conditions plant nutrient uptake is ferocious and levels of ammonia, nitrite and nitrate are always virtually zero. In addition, as plants will uptake ammonia in preference to nitrate for their nitrogen requirements, they probably offer a much more flexible response than a bacterial filter (which I also employ) to a potentially devastating sudden increase in ammonia levels. In terms of plant species, I work on the principal that plants with the least well defined root structures will be most interested in nutrient removal from the water column, i.e through the leaves rather than from the substrate.