A fish-related question

From my understanding, theyre not chemically changing the water. Water contains dissolved gasses, the same gasses that are in air as far as i know. The fish's gills filter out the dissolved oxygen from the water. I know when i took Aquatech in high school we took Dissolved O2 samples from various water samples, so im fairly sure thats how it works. Been a couple years since i had that class, and 4 years since Bio.

So to answer your question; they breathe in water, and dissolved gasses, and breathe out water and dissolved gasses, however the dissolved gasses would contain more CO2.