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What is worse: permanently high CO2 or dramatic pH swing?

M

Marcel G

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Hi all,

recently I do some pH monitoring in my tanks with CO2, and have some questions which I can't find answer for.

I have read some [non-scientific] articles where the authors say that rapid pH changes cause stress to fish. When I measured the pH in my tank during 24 hours period, I have find out that the pH droped by 1 point (from 7.4 to 6.4 in my first tank, or from 8.0 to 6.9 in my second tank) in just 2 hours after I turn the CO2 system on/off ! Is it true that these shifts in pH could cause stress to fish/shrimps? Does anyone know some guaranteed info on this, or link to scientific study?

Also some people don't turn their CO2 supply off at night to keep the pH relatively constant (to avoid these dramatic pH swings). But this way they have permanently high CO2 levels (although they keep them a little lower => say around 15-20 mg/L). Many authors state concentrations above 20-30 mg/L as dengerous for fish (e.g. Christel Kasselmann, Horst and Kipper, Ines Scheurmann, George Booth, Chuck Gadd).

Both seems to pose some problems or risks for fish and shrimps:
1) Permanently high levels of CO2 could cause acute or even chronic stress to fish.
2) Dramatic pH swings can cause stress or even death to fish.

Are both statements true?
And if so, what is worse?
Is it better to have permanently high CO2 level with stable [non shifting] pH, or to have high CO2 level at day, and low at nigh ... but with dramatic pH swings?

Marcel
 
Hi Marcel,
We have plenty of empirical evidence to demonstrate that cyclic pH changes or rapid pH changes due to CO2 injection is not harmful. I think people put the "cart before the horse" when it comes to pH. The danger posed by a pH change depends entirely on WHY the pH is changing. If the pH falls or rises rapidly due to the release of a toxic agent in the water column then the toxicity will be due to the effects of the toxic agent, not due to the pH. So for example if there is ammonia in the water and if the pH is low then this reduces the toxicity of the ammonia because of the higher ratio of the less toxic NH4+ to the more toxic NH3. If the pH were to suddenly increase dramatically then this would change the balance in favor of NH3 and so the condition of the water would be toxic - but this is due to an increase in NH3 and a reduction of NH4.

Conversely, if you were to add a strong acid to the tank this would result in a dramatic fall of the pH, but the toxicity will be due to the toxic acid, not just due to the pH. Strong acids disassociate to release massive quantities of H+ in the water column. These strong positive charges rips electrons from the orbital shells of tissue molecules and THAT'S why strong acids are dangerous, and that's why they are used to generate electricity in batteries and so forth.

CO2 is a very weak acid and so, even though it releases H+ into the water, the quantity of H+ is very low. This causes the pH to drop dramatically but does not do harm to tissue structure. That's why, for example, you can eat vinegar, which is a weak acetic acid, but you would think twice about putting battery acid on your salad, right?

The dangers of permanently high CO2 in the water depends entirely on how high the level is. The effect of hypercapnia (high CO2 content) is that the high partial pressure of the CO2 in the water column, restricts the flow of CO2 from the fish's bloodstream into the water, so the CO2 concentration builds in the blood. This causes the blood to acidify and that becomes toxic. high bloodstream acidity reduces the ability of hemoglobin to hold Oxygen. The fish combat this by increasing the production of bicarbonate (HCO3-) to reduce the effects of the acid. However, there is a threshold whereby the bloodstream cannot produce enough HCO3- to buffer the blood. As the level of CO2 and blood acidity rises, the efficiency of the hemoglobin falls dramatically and the acidity also affects the function of nerve synapses, so the nervous system is endangered.

So it's not a black-or-white choice between permanently high CO2 versus pH swings. One really has to understand the root causes as well as the concentration levels of the chemical agents affecting the pH or being affected by the pH. Either path has danger and so the danger must be assessed and managed.

Cheers,
 
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