# pH, CO2, and fish - why they gasp when CO2 increases



## Zapins (Jul 28, 2004)

Just found some interesting info in a text book while studying for animal physiology.

"The effect of acid on the dissociation curve is much more pronounced in some fish than in mammals; this stronger effect is known as the Root effect. The most significant consequence of the Root effect has to do with the function of the blood in the secretion of oxygen into the swimbladder (Pelster and Weber 1991).

The peculiarity of the Root effect is that the blood is not fully saturated with oxygen at low pH, however high the oxygen pressure. This is explained by the fact that fish blood contains two different hemoglobin species, one highly sensitive to acid and the other acid-insensitive. In the presence of high carbon dioxide the former carries no oxygen, while the latter presents the usual dissociation curve for this hemoglobin species (Gillen and Riggs 1973).

Why do fish have two different hemoglobins? It is easy to see the value of the acid-sensitive species, but why have another kind that is acid-insensitive? Isn't this unnecessary?

The explanation is that when a fish is in an emergency it is making a maximal swimming effort, it produces a great deal of lactic acid. If the presence of this acid in the blood made all the hemoglobin incapable of binding oxygen, the fish would simply die of asphyxia. However, the presence of an acid-insensitive hemoglobin avoids this calamity."

Soo, when we have the CO2 going crazy at full blast and put fish that haven't had time to adapt to high CO2 concentrations and the lower pH we are effectively stressing it's body in the same way that it would be stressed if it were swimming away from predators at full stick. Like running a marathon...


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## Zapins (Jul 28, 2004)

Hmm... so does the fish eventually die/pass-out from exhaustion or does it die from something else that the CO2 does to its body?


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## Goat (Mar 14, 2008)

I had a small clown loach (2" or so) that started doing flips and going crazy until he'd have a seizure. I did a water change immediately, as the CO2 got too high, and it died a couple days later. But he did that constantly all day every time I got near the tank, he would freak out so bad he'd pass out. Although he had always been in the tank since the CO2 was added. Interesting article.


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## Zapins (Jul 28, 2004)

I just read somewhere else that when CO2 is at very high concentration in the water the CO2 made inside the fish from normal metabolic activity can't diffuse out to the surrounding water because the partial pressure of CO2 outside the fish is much higher than inside, so the CO2 gas would have a tendency to flow into the fish making it worse off for a number of reasons.

One: 
The acid concentration goes up deactivating 50% of the haemoglobin (stressing the fish out because its essentially getting half the O2 delivered to its tissues).

Two: 
The acid (H+ ions) irritate the fish's gills so that extra mucus is produced. If the acid in the water is quite strong then the mucus will keep being produced and thicken to the point where it prevents O2 from entering at a fast enough rate to keep the fish alive. This is definitely the primary reason fish start breathing quickly.

A little biology
Fish, unlike most land creatures, are stimulated to breath by a lack of oxygen rather than an increase in CO2 concentration. This makes sense because CO2 in the water generally never exceeds ambient conditions (3-4 ppm) since it turns into carbonic acid shortly after entering the water. This is because water can hold roughly 10 times as much carbonic acid than it can hold of CO2 gas, it makes little sense to have the fish sensitive to an increase of CO2 gas if the constant surrounding concentration is 3-4 ppm. If the fish's only stimulus to breath was an increase of CO2 gas in the water then the second breath might never come since most of the CO2 in the water is held in carbonic acid form and this doesn't change very fast. Rather than CO2 being the stimulus to breath a lack of O2 makes far more sense because O2 doesn't dissolve into water very fast, staying at roughly 8 ppm, it is quickly depleted by aerobic bacteria/other oxygen consuming creatures. Therefore oxygen levels change much more drastically than CO2 levels do in water and this is why fish are sensitive to it over CO2. The lack of O2 absorption from a thick mucus lining causes the fish to respire much faster than normal.

Creatures that breath air directly don't have water surrounding their lungs that can absorb CO2, instead the CO2 stays as a gas (that can build up and cause problems), thus it makes more sense for breathing creatures to be stimulated to breath by an increase in CO2 concentration. Additionally, the air always contains ~22% oxygen so being sensitive to a decrease in O2 doesn't make any sense at all for air breathers since O2 concentration in air is the same from sea level to the brink of space.

Three: 
Though I haven't fully pieced together the specifics of it, extra acid in the water enters the blood and changes the normal concentrations of positive ions like Ca and Na. Difficulty maintaining ionic concentrations in the blood and tissues cause problems with osmoregulation.

The fish soon isn't able to compensate for the changes and dies from the combined stress of all three reasons.

One of the websites I used for some of the basic info is quoted below except instead of carbonic acid they are referring to sulphuric acid. The difference in acid type makes little difference since acid is a measure of H+ ions, and all H+ ions from whatever kind of acid are the same, the only difference between the two acids is that sulphuric acid has more H+ ions to give up than carbonic acid and does so much more readily than carbonic acid.



Website said:


> Subsequently, sulphuric acid in water can affect the fish in the lakes in two ways: directly and indirectly. Sulphuric acid (H2SO4) directly interferes with the fish's ability to take in oxygen, salt and nutrients needed to stay alive. For freshwater fish, maintaining osmoregulation is key in their survival. Osmoregulation is the process of maintaining the delicate balance of salts and minerals in their tissues. Acid molecules in the water cause mucus to form in their gills and this prevents the fish to absorb oxygen as well. If the buildup of mucus increases, the fish would suffocate. In addition, a low pH will throw off the balance of salts in the fish tissue. Salts levels such as the calcium (Ca+2) levels of some fish cannot be maintained due to pH change. This results in poor reproduction - their eggs produced would be damaged; they are either too brittle or too weak. Decreased Ca+2 levels also result in weak spines and deformities. For example, crayfish need Ca+2 to maintain a healthy exoskeleton; low Ca+2 levels would mean a weak exoskeleton. Another type of salt N+ also influences the well-being of the fish. As nitrogen-containing fertilizers are washed off into the lakes, the nitrogen stimulates the growth of algae, which logically would mean an increase in oxygen production, thus benefitting the fish. However, because of increased deaths in the fish population due to acid rain, the decomposition process uses up a lot of the oxygen, which leaves less for the surviving fish to take in.


Taken from:
http://www.geocities.com/capecanaveral/hall/9111/DOC.HTML

Another website with a small paragraph about high external CO2 concentrations blocking the diffusion of CO2 from the body is below:
http://www.hallman.org/plant/booth1.html


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## YankyTexan (May 7, 2008)

Extremely interesting article/information. Thank you for taking the time to compose it and share with the rest of us.

Given this information what conclusions and/or best practices can be formulated? In an all-out effort to grow plants at an unnaturally fast rate (ie. pressurized Co2) are we slowly stressing our fish?


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## CherylH (Jul 22, 2004)

I'm finding this series of posts very interesting. 

I think we do have a tendency to put our plant health above the health of our fish and look at plant growth instead of the health of the entire system. Some of the reading I've done indicates that higher CO2 levels are more unhealthy for the fish if O2 levels are low.


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## hoppycalif (Apr 7, 2005)

Very interesting. I don't know any of the answers about CO2 toxicity for fish. I don't doubt that our abnormal levels of CO2 in the water are stressful for fish, but if kept within reason the fish seem to manage that stress well, since so many of us have long lived fish in our tanks. I keep an aquarium primarily for the plants, so the "comfort" of the plants is a higher priority for me. Others have different goals and standards.


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## Zapins (Jul 28, 2004)

Given this information what conclusions and/or best practices can be formulated? In an all-out effort to grow plants at an unnaturally fast rate (ie. pressurized Co2) are we slowly stressing our fish?

No problem YankyTexan, and CherylH  it is interesting and enjoyable for me to research this stuff as well as share it with everyone.

I think the best practices are not to over do the CO2 levels, or if an extremely high CO2 level is needed it is probably better to remove the fish, or at the very worst raise the CO2 concentration slowly over a period of days or weeks so the fish can adapt to the change and make the physiological adaptations that are needed. If you raise the CO2 slowly the fish will adapt rather than show stress symptoms (which happen because the compensation mechanisms can’t fully compensate for a huge change in a short time frame). 

I don’t think the fish are very stressed out from the addition of CO2, anywhere up to 30 ppm won’t really harm them, it might make the fish breath a little faster in the beginning, but then they adapt and don’t appear to be in any stress. 

It might be because the fish are able to produce more red blood cells that can absorb more O2, or maybe the gills become less sensitive to acid. I’ll have to look into it a little later on.

As hoppy mentioned I also keep aquariums primarily for the plants (the fish are only there to stop people from asking why my “fish”tank has no “fish” in it). I don’t mind stressing the fish a little initially for the sake of the plants. If you think about it, the fish probably gain other health benefits from having healthy plants all around them they feel more secure – hiding places, more oxygen in the water, lots of breeding sites, etc… It probably is an equal trade off in the end.


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## geeks_15 (Dec 9, 2006)

In humans too much CO2 leads to hypercarbia. The blood becomes too acidic (acidosis). The first human physiologic response is to increase respiration and therefore exhale more CO2. This can compensate for a time and raise the pH of the blood. Here is the chemical formula.

H+ + HCO3 <-> H2CO3 <-> CO2 + H20

So when you add CO2 it pushes the formula to the left and more H+ is released and the pH drops. PH is basically a measure of the concentration of H+ in solution. If you eliminate CO2 the formula shifts back to the right and free H+ is decreased. Human blood is a complex mixture of electrolytes and proteins in a delicate balance. If the pH is too acidic or basic it starts to throw things off.

I wouldn't be surprised if fish are doing the same thing. Increase respiration to eliminate CO2 from their bodies.

The mucous on the gills theory is interesting. I wouldn't expect acid loving fish to have this problem though. I have never heard of apistos or tetras being immune to too much CO2 in the tank.


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## geeks_15 (Dec 9, 2006)

To follow up (sorry more human physiology):

Humans can develop disorders that create a chronic acidosis (blood more acidic than the normal human range 7.35 to 7.45). One is COPD where CO2 cannot be exhaled effectively and they get a chronic build up of CO2.

Long term compensation (increased respiration works in the short term, but not long term) occurs when the kidneys excrete H+ and reabsorb bicarbonate (HCO3). This raises the pH of the blood.

Maybe fish are doing the same thing or something similar.


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