To the best of my knowledge fish use they're gills for this purpose . Jim123
How Do Fish Get Rid Of Carbon Dioxide?
It becomes dissolved into the water. If the water is saturated then it is released into the air.
Using It's Gills... When It Breathes In, The Water Will Be Sucked In With The Air And Will Be Released With The Water Itself.
Try passing it through sodium hydroxide this will give sodium carbonate and water and thus CO2 is vanished
CO2+NaOH => NaCO3+H2O
CO2+NaOH => NaCO3+H2O