Unfortunately, there are many ways water is polluted, here are a few:
Raw sewage dumped into water
Agricultural spill offs (If the ground is over fertilized, it can spill off into a body of water during rain, which cause a massive algae bloom and can kill lots fish. This is because the algae blocks the sun, limiting the amount of sun the plants get. A by product of photosynthesis is oxygen, so the less oxygen, the less fish can breath. this means a lot of fish will suffocate to death.)
