The amount of carbon in the aquarium's water started out high. Now, the amount of carbon in the water is decreasing. Is the aquarium now in sunlight or is it covered? What is happening to the number of energy storage molecules in the plants and fish as a result?

Ithink a saturated phospholipid

Mitochondria produce atp, a high energy compound. atp is used for all sorts of enzyme-mediated reactions, such as making proteins/nucleotides/etc, certain compounds cross the cell membrane, muscles move, etc.  without this source of energy, a cell will die. it will not be able to repair itself, defend against pathogens, etc.