From many posts here, it seems that high CO2 levels make the fish come to the top of the water and "gasp" for air.

Why does this happen? I don't think that a high CO2 level "drives out" the O2. Could it have something to do with the fact that the water passing through the gills has more PPM's of dissolved gas with the higher CO2, and the gills don't distinguish between that and O2, thus reducing the amount of O2 in the fish's bloodstream?

Thanks.

Bill

From a layman's perspective, and by that I mean a layman with negligable knowledge about the physical processes in fish, I think CO2 being too high prevents fish from eliminating the CO2 in their blood. Logic tells me the ppm of CO2 in the water has to be significantly lower than than in venous blood in the fish, or no CO2 can be transferred from the blood to the water.

Thanks!

I know that osmosis occurs when the density of a substance on one side of the semi-permeable membrane is less than that on the other side, so the fluid flows from the less dense side to the more dense. Is the elimination of CO2 from the fish's bloodstream based on something like that?

I am also wondering about the concentration of CO2 in the bloodstream and it's relationship to that in the water column. If the fish is OK at a CO2 of 40 PPM in the water and not OK if that concentration is 90 PPM, does that suggest that the blood concentration is between 40 and 90 PPM? It would seem to follow.

And, to introduce a bit of levity, at times my CO2 levels rise due to drinking beer. That is usually eliminated in a natural manner. Can fish do that?

Bill

My Yoyo Loaches seem to be eliminating excess gas of some kind, because I hear little noises as they get involved in eating. I don't serve beer though.

EDIT: It occurred to me that I might come across as a prude, but that isn't the case at all. I simply believe in obeying the laws that require someone be over 18 to be served beer. Not one of my tenants in my tank even approaches that age.

Are CO2 and O2 concentrations really totally independent of each other?

Also, wouldn't the water closer to the surface have less co2 in it simply due to the fact that it's closer physically to the point where equilibrium is taking place with the atmosphere? If so, the fish are just trying to get away from the co2, not get more O2.

That is a good point about the CO2 concentration being lower at the top of the tank. Logic says there has to be a gradient in concentration of CO2 in the water to get a flow of CO2 out of the water at the top. I had never considered that. Perhaps that gradient is very small when we have good water circulation?

I'm not sure what the answer is but it doesn't seem reasonable at all to me that CO2 concentration is lower at the top of the tank. It's the lower regions of the tank that suffer BBA most frequently and in deep tanks it's the carpet plants that appear to be most limited. Any CO2 mist or bubbles tend to rise as they exit the injector so it seems to me that if anything, it's the upper regions which would tend to have a higher concentration. It also seems that if a fish gulps a bubble of air and passes it over the gills, that air bubble would tend to have a lower CO2 concentration than it's blood facilitating the osmosis of the CO2 from blood to bubble. I suppose it could be crudely tested by stacking an array of drop checkers vertically and observing the color change profile of each.

Cheers,

Carissa raises a very good question.

I am wondering if the circulation in the tank would not be enough to keep moving the CO2 around in the tank and thus maintain a more-or-less steady level in all parts of it?

If that didn't happen, the carpet plants wouldn't get enough CO2, right?

Bill

It has to take a finite time for CO2 in solution to move a certain distance in the water, from higher concentration to lower concentration. That would support there being a gradient of concentration from top to bottom. Also, CO2 is in the tank so the plants can absorb it as a nutrient. Immediately around a plant leaf there has to be a gradient in CO2 concentration, low at the leaf surface and higher farther away. Fast growing plants must absorb nearly as much CO2 as we add - a few "bubbles" per miinute. I have no idea what the speed of dissolved CO2 in the water is, as it moves from high to low concentration areas. If it is sonic velocity, the concentration would have to be almost totally uniform throughout the water, but if it is inches per minute, there have to be large gradients in concentration.

Makes sense about the fish gasping for air bubbles. I'm not sure whether fish are just hanging out near the surface or taking in bubbles, I've never had a co2 overdose before to observe what actually happens.

It's true that co2 bubbles rise, but what we are measuring are not the visible bubbles, it's the concentration of dissolved co2. Just because bubbles rise doesn't mean that co2 concentrations would be higher nearer the top. The bubbles usually start off larger at the bottom and by the time the get to the top, they have shrunk in size, meaning they have mostly dissolved, having left behind most of their co2 before they even get to the top.