Hi all,
Julia as the other posters have said they are quite delicate fish, but as they come from very warm, still waters they will not have a very high oxygen demand, and therefore they won't be any more sensitive to CO2 than other fish. For example warm water can hold less oxygen than cool water.
If you think of the CO2 and O2 exchange surfaces in the fishes gills, it is the differential between the gas levels across the exchange surface that matters, in a highly rheophilic fish from cooler waters (Salmon, Hill-stream Loach or Chaetosoma) the fish has evolved in a situation where a constant stream of water, rich in oxygen, has flowed across it's gills, allowing rapid diffusion of CO2 and O2 in. Problems are likely to occur when that fish is placed in warmer or less oxygen rich water as the rate of gas transfer slows. Fish from very oxygen depleted water often have additional methods of obtaining oxygen, labyrinth organ in Bettas, Gouramis etc. oxygen gulping and modified gut in Corydoras, some Plecs etc.
cheers Darrel