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Pelvicachromis

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78
This thread is exactly why I am happy that apistogramma.com exist! Thanks to all involved in the discussion. And hopes that the OP post soon that he has tons of healthy fry!

Keith Chitwood
 

dw1305

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Hi all,
Ted, I'm not advocating the use of CO2, quite the opposite, personally I can't envisage ever going down the EI route.

This should be available, and pretty much sums up my thinking on O2/CO2/biological filtration etc. <http://plecoplanet.com/?page_id=829>.

cheers Darrel
 

wethumbs

Active Member
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476
Interesting to see Dave in Sister, OR has the same pH issue I have here. The high pH in the water is due to NaOH the city use to buffer the water. One major drawback with NaOH is it tends to collect at dead ends of the water line, or anywhere there is low water usage like a new subdivision (in my case)that are not fully developed. I have call the supervisor in charge of the muncipal water and he was reading the pH values from various point along the water system. Needless to say, the pH values were all lower than what I measured from the tap. His solution to my high pH is a periodic flush of the line by opening a fire hydrant. In fact, his record indicated that it was done about 6 months ago and due for another scheduled flush.

I find it puzzling that the adoketa fry would die in a couple of day. Somehow, I dont think the use of H2SO4 has anything to do with it. To test this, you can switch out the H2SO4 with H3PO4 to lower the pH, which basically replaces the sulfate with a phosphate.
 

dw1305

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Hi all,
The high pH in the water is due to NaOH
The water companies do the same thing in the UK, they inject NaOH and add a phosphate compound, the aim of this is to reduce the likelihood of the water gaining lead or copper ions from the pipe work. Technical term is "PIMS", phosphate induced metal stabilisation for control of "plumbosolvency". <http://www.edie.net/library/view_article.asp?id=1814>.

NaOH the city use to buffer the water
I know what you mean, but we have to be very careful with the term "buffer", as it has a precise chemical meaning <http://en.wikipedia.org/wiki/Buffer_solution>. This should really should read:
"the NaOH the city use to raise the pH".
Because NaOH disassociates completely into Na+ and OH- ions, it doesn't have any buffering, and as soon as those OH- ions are exhausted there is no reservoir of them (the buffer) and the pH will fall. Again with apologies for the cross-post, but this thread deals with a very similar situation. <http://www.plecoplanet.com/forum/showthread.php?t=8904>

I find it puzzling that the adoketa fry would die in a couple of day. Somehow, I dont think the use of H2SO4 has anything to do with it. To test this, you can switch out the H2SO4 with H3PO4 to lower the pH, which basically replaces the sulfate with a phosphate.
I don't think the H2SO4 had any specific toxic effect either, I think it is do with the lack of DOC from not having humic compounds etc. The DOC will chelate trace heavy metals and possibly interact with any trace pesticides that might be present in the water, and that the fry may be especially sensitive to. If it is a chelation effect a water conditioner with EDTA should also reduce fry death, although this wouldn't work if there was lots of iron ions (Fe3+) present (FeEDTA is the most stable form).

Another possibility is that the addition of Indian Almond/Oak leaves etc adds either trace compounds (vitamins? micronutrients?) the fry may need, or possibly provides a substrate for rotifers, diatoms, protista etc, that are then consumed by the fry, and supply whatever is missing from their diet.

....... A solution at a given temperature has a limited capacity to contain gas (cooler liquids can contain more). So you cannot simply increase the aeration when adding CO2 in order to increase oxygen content...... One of the reasons CO2 causes problems for fish is that it is MORE soluble in water than O2 is. Whenever a gas has the ability to chemically combine with another molecule in the solution (in this case CO2 is combining the the water itself), the solution can hold more of the gas. CO2 + H2O = H2CO3 (carbonic acid) O2 does not have the ability to bond with water, so it is not as 'soluble'. The attraction between CO2 and H2O forces an unequal rate of diffusion of gas into the water. That means that if CO2 and O2 are presence above the water in equal concentrations, the CO2 will diffuse in faster than the O2 will... resulting in a greater concentration of CO2 than O2. The reason we 'inject' CO2 into planted tanks is that the atmospheric concentration of CO2 is naturally much much lower than that of O2.

As it was explained to me, I think this is mainly correct, but with the proviso that 20ppm of CO2 is not enough to fully saturate the water with gas, and realistically we can look at the concentration of O2 and CO2 as unrelated, (at sea level, at 27oC, pure H2O is saturated with 7.9 mg/l O2, but with only 0.42 mg/l CO2, as CO2 forms only 0.04% of the atmosphere). Where the levels of CO2 and O2 are relevant is in the diffusion gradients at the fishes gills, where it is the difference between the levels of gases in the fishes blood and the water governs the speed at which diffusion occurs. If you have high levels of O2 in the water the diffusion gradient is steeper and you can get away with higher levels of CO2 in the water.

I've no experience of whether this is true or not, or makes any sense in fish physiology terms.

cheers Darrel
 

tjudy

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My understanding of the way gases behave in water is that they do not actually dissolve into it like a solid solute does. NaCl dissolves into its ions. O2 does not dissolve into free oxygen atoms. Nor does CO2. There is a physical limit to the amount of gas that a volume of water can hold, depending upon temperature and pressure. When we are looking at a mixture of gases diffusing into the liquid, the rate of diffusion of each is determined by their concentrations. Each gas in a mixture makes up a 'part of the gas pressure' and is described as the gas' 'partial pressure'. Partial pressure is really a measure of percentage of the whole, and tells us how much of the mixture is composed of each gas. If you increase the number of gas molecules of one type of gas in a mixture, the partial pressure of that gas goes up while the partial pressures of the other gases go down. If the space where this is happening is completely enclosed and no gas can escape, the actual gas pressure of the mixture will increase (because you are adding gas molecules and volume is not changing).

If there is a way for gas to escape, however, gas will leave the volume/space where it is being displaced from as new gas molecules are added. Think of the water in an aquarium as being a fix volume space (which it is) that can only hold a fixed amount of gas molecules. When the partial pressure of CO2 in the water is increased, the partial pressure of oxygen must go down. The fish gills need a specific level of oxygen partial pressure for the gas to pass across the gills. When we inject CO2 and lower the O2 partial pressure in the water, we risk dropping the concentration of O2 below that threshold and asphyxiating the fish. The actual number of oxygen molecules may not change, but it's partial pressure will. If too much gas is injected, the O2 (and all the other gases) will begin to diffuse out of the solution as bubbles... but that simple means that the liquid cannot hold any more gas.
 

dw1305

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Wiltshire UK
Hi all,
Ok, Ted that makes sense because this is "Henry's law" and the "common ion" effect, <http://en.wikipedia.org/wiki/Henry%27s_law>
At a constant temperature, the amount of a given gas that dissolves in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid.
So this applies to oxygen, but not to CO2 because it doesn't behave in accordance with Henry's Law, as some proportion of it will ionize according to the carbonate/carbonic acid equilibrium. The maximum solubility of CO2 is 1,700ppm.

H2O + CO2 « » H+ + HCO3-

fig4-4_aeration.jpg
pH is the X axis.

Unfortunately I don't know enough about water physical chemistry to go any further, but hopefully some-one else will. You can definitely reach 100% saturation of oxygen, fairly easily, I was calibrating a dissolved oxygen meter on friday in the lab, and the tank in the lab. (2' heavily planted with the 48W 10,000K T5 lights was at 100% at 27oC ~ 8mg/l DO2).

My suspicion would be that the 20ppm CO2 is relatively irrelevant in this, but that really is only a guess.

cheers Darrel
 

tjudy

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20 ppm will not put a dent in the 700 ppm saturation, but there is already some CO2 in the aquarium from the atmosphere. Whenever I bother to test the levels of CO2 in my aquariums, it is always at the low end of the ppm scale considered adequate for growing plants. I inject CO2 into one large aquarium with a lot of plants, because within 24 hours of a water change the CO2 levels were testing below that adequate range (I would need to go search Tom Barr's website to find that range again).

If the concentration of CO2 does not affect the concentration of O2, then we would never reach a point where the fish asphyxiate when injecting CO2... but that happens, and the damage cannot be attributed to pH drop alone. I have done some diagnostic problem solving on a planted tank with gasping fish. The CO2 being injected was increasing CO2 significantly and the keeper was not turning the CO2 off at night. He would wake up in the morning to fish gasping at the surface. Within a few hours of the light being on, all was good. His pH in the morning was measuring 7.5, because his KH was high enough that the pH drop from CO2 injection was contained by buffering. Could it be that the CO2 that reacts with water does not follow Henry's Law, but not all of the CO2 combines with water? And that the CO2 that does not react to make H2CO3 does follow Henry's Law and thus result in a reduction of the partial pressure of O2 in the water?
 

gerald

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I dont know to what extent (if any) CO2 injection in an aquarium physically displaces O2, but CO2 above approx 30 ppm can be toxic to fish (varies among species) even when O2 is at full saturation (approx 8 ppm at 75F). I read somewhere (wish I could find the source again) that fish left too long in a bag usually die from CO2 poisoning (self-generated) before O2 depletion kills them.

Also, bubbles of injected CO2 are at higher pressure than the air above the tank, and a CO2-injected aquarium is NOT at equilibrium -- something to keep in mind when using gas physics "laws" to understand what's happening.
 

dw1305

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Wiltshire UK
Hi all,
I'm way from home, but when I'm back I'll try and find Tom Barr's email. I can do the amount of carbonic acid, it is very small with most of the CO2 remaing as CO2. I'd have to check, but it is something like 400:1 CO2: H2CO3.

cheers Darrel
 

regani

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429
Location
Brisbane, Australia
If the concentration of CO2 does not affect the concentration of O2, then we would never reach a point where the fish asphyxiate when injecting CO2... but that happens, and the damage cannot be attributed to pH drop alone. I have done some diagnostic problem solving on a planted tank with gasping fish. The CO2 being injected was increasing CO2 significantly and the keeper was not turning the CO2 off at night. He would wake up in the morning to fish gasping at the surface. Within a few hours of the light being on, all was good. His pH in the morning was measuring 7.5, because his KH was high enough that the pH drop from CO2 injection was contained by buffering. Could it be that the CO2 that reacts with water does not follow Henry's Law, but not all of the CO2 combines with water? And that the CO2 that does not react to make H2CO3 does follow Henry's Law and thus result in a reduction of the partial pressure of O2 in the water?

the problem you are discribing is not related to O2 being replaced by CO2 as some from of gaseous exchange, I think. in the example you are describing the different CO2 and O2 levels are more likely due to the metabolism happening in the plants. with the lights on the plants will consume some of the CO2 and producing O2 which dissolves into the water and providing a certain ratio of CO2 to O2 once equilibrium is reached. with the lights off the plants are switching their metabolism away from photosynthesis and they are 'burning' nutrients using O2 and generating CO2, similar to normal metabolism in animals. so over night a different equilibrium is established with higher CO2 and lower O2.

don't forget that not only lower oxygen levles are problematic, but that CO2 is also toxic itself (not as much as CO, but still...); even with constant O2 levels rising CO2 will lead to asphixiation as it messes with the oxygen transport in the blood and tissue

I am not too firm on all the physics (it has been a loooong time since I had to learn all that stuff), but I think you are correct when you suspect that CO2 doesn't behave like a normal gas when it comes to mixing with water because it is not inert but forms carbonic acid, so it can reach much higher concentrations in water that other 'inert' gasses like oxyg or nitrogen.

Also, bubbles of injected CO2 are at higher pressure than the air above the tank, and a CO2-injected aquarium is NOT at equilibrium

the pressure in the CO2 bubbles is actually not much higher that atmospheric pressure. the pressure inside the bubble is determined by the depth it is in with the water column 'pressing' from the outside. what is different, however, is the partial CO2 pressure inside the bubble compared to the atmosphere, i.e. the concentration of CO2 in the gas making up the bubble. the atmosphere has only a very small percentage of CO2 (approx. 0.39%) compared to almost 100% in the injected CO2. so we are looking at two different situations, one at the water surface with low CO2 concentrations, and one at the surface of the bubbles with very high CO2 concentrations. if everthing stays the same (rate of bubbles, surface agitation, concentrations of verything else in the water) we will arrive at a new equilibrium with a certain concentration of CO2 in the water that is specific for each particular system we are looking at.

as mentioned above things like plants and fish will have an impact, but for any given tank with a fixed routine of feeding and maintenance the changes of concentrations of CO2 and O2 in the water should largely follow a particular pattern and not deviate too much - unless something happens
 

jaafaman

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Messages
40
Location
Chattanooga, Tennessee
The partial pressure of oxygen is not reduced simply because CO2 is injected - only its total percentage of the entire makeup of the solution changes. Nor does the concentration of oxygen in the water get replaced by the CO2 that is injected in any form of displacement. Acidosis within the bloodstream is the causal effect here, since a higher concentration of CO2 will cause the hemoglobin to retain more of the gas because there is not a sufficient pressure differential to expel the excess. While the renal system fights to maintain ionic salt balances, the respiratory system is the only means of maintaining gaseous levels within the bloodstream. As acidity increases, biological processes that have evolved to occur within specific pH ranges are decidedly affected, most noticeably protein and amino acid assimilation as well as the conversion of ADP to ATP and its disposal since the blood cell itself can only carry so many molecules and its oxygen content can definitely be affected by displacement.

Try it on yourself in a reversal of the situation through what's known as "hyperventilation", where neither the oxygen nor the CO2 content of the air you're breathing changes - only the way the gasses are exchanged in your system. Breath fast enough and shallow enough and you'll reverse the situation to where there's nore than enough oxygen in your bloodstream but the CO2 is reduced to much closer to that of the atmosphere's pressure as your "open system" seeks equilibrium. You'll find that an increase in alkalinity will pretty much affect your body in the same way.

CO2 becomes much more lethal in the atmosphere than it does in water, as its weight allows it to "pool" in low areas that folks like spelunkers are more apt to frequent. In these cases, the immediate danger is indeed the total displacement of oxygen by CO2 and death occurs through simple ashpyxiation through anoxia...
 

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