Throw away any left over juices or grease, including the water in which the fish was boiled or poached. Do NOT use them to make a soup or stock. It is never possible to remove all of the chemicals from a fish. Choosing safer types of fish and less contaminated places to catch them is the best way to reduce potentially harmful chemicals in fish you eat. Safe varieties of fish, caught in safe areas, are generally considered safe fried and with the skin left on.
All fish contain some chemicals.
Larger and older fish build up the chemicals from each fish they eat. And we build up chemicals from each fish we eat. Fish found in local lakes and rivers may contain dangerous chemicals. Chemicals are stored in either the fat or the meat of the fish. Fattier fish and fish that eat other fish, especially when larger or older, can build up high levels of chemicals.