CO2 and Fish

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.

 

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,

 

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.