Respiratory gas exchange is a two-way process: CO2 diffuses out of the body as O2 diffuses in. The direction and rate of diffusion of the respiratory gases across the exchange surfaces depend on the partial pressure gradients of the gases. The partial pressure gradients of O2 and CO2 across these exchange surfaces are quite different. The amount of CO2 in the atmosphere is extremely low (0.03%), so for air-breathing animals there is always a large concentration gradient for diffusion of CO2 from the body to the environment. Whereas the partial pressure gradient for O2 decreases with increasing altitude, the gradient driving CO2 out of the body hardly changes. The partial pressure of carbon dioxide (PCO2) in the atmosphere is close to zero both at sea level and atop Mount Everest.
In general, getting rid of CO2 is not a problem for water-breathing animals because CO2 is much more soluble in water than is O2. Even in stagnant water, where the PCO2 is higher than in moving water, the lack of O2 becomes a problem for an animal long before CO2 exchange difficulties arise.