Chemical pollution is when toxic fluids get released into the water. Sometimes factories and industry will allow waste products to flow into rivers which eventually get to the sea. Sometimes chemicals are dumped at sea. Chemicals are used in millions of processes like making food, plastic toys or electricals.
Through rivers and streams those chemical enter marine food chains Sometimes medicines that humans and animals take also contain bad chemicals and these make their way into the sea through our toilets, and then into rivers and into the ocean.
Toxic chemicals in the ocean get to plants. Fish then eat those contaminated plants and die. Even if fish don’t die, they can get very very sick.