Go through the process of RESPIRATION.
The gills of a fish release carbon dioxide into the water. A sugar that enters the fish in its food and used to build its muscles gets burned for the energy for the fish to flip its tail. The resulting carbon dioxide gets carried in the bloodstream to the gills. The released carbon dioxide remains suspended in the water. That sugar came into the fish through its food. The oxygen needed to burn the sugar came into through the gills in exchange for respired carbon dioxide. That oxygen travelled through the bloodstream to the muscle where the sugar was burned. , oxygen is exchanged with carbon dioxide. Water with dissolved oxygen in it is pulled in through the gills of a fish. The oxygen is exchanged with carbon dioxide in a process of diffusion. The oxygen is carried by blood to other parts of the body, where it reacts with organic carbon compounds to release the energy needed for life.

[In as food; out as carbon dioxide]

Go through the process of DECOMPOSITION.
Once a fish dies it becomes food for microorganisms. Dead The remains of marine animals, rich in carbon, provide energy for other organisms in the food web. As the remains float about or begin to sink to the ocean floor they are broken down. Microbes decompose the animal bodies, break carbon bonds for chemical energy, leaving smaller and smaller bits of nutrients, and carbon dioxide,and carbon dioxide suspended in the water. Small bits of the organic matter clump together and fall to the ocean floor as "marine snow." Eventually, pieces of plants and animals pile up on each other, adding more layers to the ocean floor. Pressure from the accumulating material and the ocean water above compounds the carbon into sedimentary rock, like limestone.

[just talk about the bits of bio matter, not the carbon dioxide part?]