Worried - Is CO2 Killing My Fish

[7Enigma]

Lots of good info in this topic. The one key missing to this discussion is the affect of low pH on the nitrifying bacteria in a tank. While fish are not harmed/affected by pH's below 6.5, the bacteria that are the sole converters of ammonia and nitrIte during the non-lighted periods definately are. It is very possible that with the lights off and the pH going below 6.5 (have you ever checked the pH right before the lights come on?) you are having small amounts of ammonia present or even worse (and more likely) nitrIte present. NitrIte is a respitory affector, similar to carbon monoxide in humans.

Couple the low pH, with the likely depleted O2 in the tank (since the bacteria are using this as well as the fish and plants), and possible small amount of nitrIte in the tank, and you might have a recipe for disaster. Please note that nitrifying bacteria stop metabolism at 2ppm of O2 or less (and the nitrIte to nitrAte converting bacteria are more susceptible to this).

This is why I always recommend keeping the KH above 3 degrees in CO2 injected tanks (especially if DIY), because if the KH is low and your routinuely getting into that danger zone of pH (close to 6.0 or less), you are taking the chance of not having that biological filter working in proper fashion.

As for the fish's blood CO2 levels it NEVER equalizes to the water column's CO2 level (unless the fish is dead). Respiration for both humans and fish is completely passive (diffusion). High levels of CO2 present in the blood after circulation will gas out into the water, while O2 will diffuse into the blood because there is a concentration gradient present. So the levels will get close to equal for a split second when the blood comes in close proximity to the water, and then will immediately go back to higher levels in the fish (CO2), and lower levels of O2.

When the CO2 level gets extremely high in the water column (say >100ppm), the fish's blood will have to become MUCH more saturated with CO2 before it will gas out. It's NOT a factor of having to respire faster, that is a misnomer. In a high CO2 environment both a human and a fish are at the mercy of the environmental CO2 concentration, there is nothing they can do to "breath" better. Faster respiration rate is actually detrimental to both humans and fish in a high CO2 environment (they simply deplete their O2 supply faster since they are using more energy).

Unfortunately for both fish and humans the biological response to high CO2 levels is faster respiration rate, but it has no benefit, until the environmental level goes down.

That's why on my tank (DIY CO2) I have the KH at 5-6 degrees, plumb the CO2 output line directly into my Aquaclear HOB filter and adjust the flow at night so more CO2 gasses off (but more importantly that O2 levels are kept high). I'm confident then that both plants and fish are safe.

Oh and as for the saturation point of CO2 in water its WELL over 1000ppm (I think somewhere closer to 1500ppm).

HTH,

justin

 

[Wizzard~Of~Ozz]

Interesting view on the nitrifying bacteria, which virtually stops at PH 6.5. However one would have to consider that the plants are more important then the bacteria. Infact, I could almost say the bacteria are detrimental to the growth of plants as they compete for Nitrogen (NH3) and Co2. Not to mention other nutrients. Most planted tanks don't really require any bacteria.

 

[7Enigma]

Interesting view on the nitrifying bacteria, which virtually stops at PH 6.5. However one would have to consider that the plants are more important then the bacteria. Infact, I could almost say the bacteria are detrimental to the growth of plants as they compete for Nitrogen (NH3) and Co2. Not to mention other nutrients. Most planted tanks don't really require any bacteria.

Rather than say they are detrimental, I would rephrase it to say they are a necessary evil.

The problem is that at night while the dark cycle is indeed active in the plants, the nitrogen uptake (we only care about ammonia for this case) is greatly decreased. The fish will continue to produce waste during this lowered activity and this could be causing stress for the fish.

Worse, if indeed there is nitrIte present due to impacting the bacteria, the plant has no use of this nitrogen source (can only be removed by a water treatment product such as Prime, or by the bacteria once the pH goes again above 6.5).

I still feel that it would be near impossible to fight algae growth if no nitrifying bacteria were present. The nitrogen (ammonia) uptake by plants is not near as quick, nor as thorough (remember the filter is cycling the water many times over per day whereas a plant has a boundary layer that is difficult to cross especially in a low flow area).

That's why people with plant only tanks (no fish or inverts) dose with nitrAte sources. While its technically easier and better for the plant to absorb ammonia as its nitrogen source (low ammonia concentrations that is), it would be a miserable (losing?) battle against algae as the leaves would be smothered by the algae, ultimately killing the plant.

 

[Wizzard~Of~Ozz]

Sorry, I was thinking in terms of larger tanks (>=40Gal) where it would take more then 12-14hrs to build up any measurable levels of ammonia. And I was also discussing a situation where dosing Nitrate is mandatory or borderline (Ie. over balanced or balanced eco system). Having an excess bioload would be hazardous in any case (a simple power outtage for 24hrs would result in floaters)

 

[LindaC]

It is very possible that with the lights off and the pH going below 6.5 (have you ever checked the pH right before the lights come on?) you are having small amounts of ammonia present or even worse (and more likely) nitrIte present. NitrIte is a respitory affector, similar to carbon monoxide in humans.

I seriously doubt this is the problem, my PH is always at 70 - 7.2 right before the lights/CO2 comes on. I believe I said this in one of my previous posts. The PH out of the tap is pretty high and I also buffer my water to bring the KH up to 70 because it's low, 40 before adding baking soda. I also have very soft water, that's at 40 too!

Thanks, I am learning so much, this post has become quite the learning tool for me.

 

[7Enigma]

OK then, we can eliminate pH as being a problem (that's a big factor to not be worried about). These type of unknowns are always a pain but if you can eliminate the variables you should be able to figure out what is going on.

 

[LindaC]

Yes, that was my biggest concern, the PH, so I'm glad that's not the reason, it might have been the Neons being introduced without putting them in QT, although that wouldn't explain the two Rams dying before that.

I keep going over it and my head and can't seem to come up with a logical explanation. Guess I'll have to chalk it up to one of those strange unexplained phenomenons.