All fish are broken down into two very broad categories — fish and shellfish. Fish without shells are separated into two groups — freshwater and saltwater. Further, fish are divided into three categories based on their fat content — lean, moderate-fat and high-fat. Fish are an excellent source of protein, B complex vitamins and minerals including calcium, iron, potassium and phosphorus. B oth saltwater and freshwater fish are low in sodium content and calories compared to meat.