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