Answer:
Oxygen
Step-by-step explanation:
Fish and aquatic animals cannot split oxygen from water (H2O) or other oxygen-containing compounds.
Oxygen levels also can be reduced through over-fertilization of water plants by run-off from farm fields containing phosphates and nitrates (the ingredients in fertilizers). Under these conditions, the numbers and size of water plants increase. Then, if the weather becomes cloudy for several days, respiring plants will use much of the available dissolved oxygen(DO). When these plants die, they become food for bacteria, which in turn multiply and use large amounts of oxygen. And this depleting all the oxygen.