Water pollution gets into the fish and kills them. But...how? Fish actually breathe water, you might say. Plus, they live in it. And so do other aquatic creatures. When living in pollution, they live, eat, sleep, breathe and drink it. How could it not hurt them? It might happen little by little if the water is not completely tainted with toxins.
That might be the cruelest of the scenarios. Then there are times they are bombarded with pollutants like when there is an oil spill in the ocean. That is when the public becomes more aware of the issue but it is usually to late to do much about it as the animals and aquatic life are already dead. That is how it is, a vicious cycle that no one is stopping soon enough.
Water pollution kills water animals through poisoning by chemical run-off, or disturbing their habitat, or using the oxygen they need to live. Sometimes the pollution is not from neaby, but has travelled hundreds of miles through the waters and then it is called transboundary pollution.
The pollution come from varied sources: surface or groundwater, microbiological, suspended matter, and chemical run-off. All are serious and that's why water is constantly tested before human use.