Summarize:

Reliable brands produce fish oil supplements with minimal degradation, which maintains the purity of the oil for maximum effectiveness in the body. It will also ensure that the ingredients in the supplement are natural and uncontaminated with harmful substances. Reputable supplement manufacturers follow certain production standards and will meet quality guidelines such as the Norwegian Medicinal Standard, the European Pharmacopoeia Standard, or the voluntary U.S. Council for Responsible Nutrition standards. Mainstream manufacturers use processing techniques to remove as much mercury as possible from their fish oil. Many brands are even able to get mercury content down to a very few parts per trillion. These are the recommended minimum daily dosages for pregnant women. Noticeable fish odor or taste may indicate that the supplement has degraded due to light exposure and gone rancid. If a supplement is scented or flavored with strong additives, it may be an indication that the manufacturer is masking a fishy smell or taste. High-quality supplements rarely have added flavors or scents, and will not taste or smell fishy. However, if you belch after taking a supplement it may smell slightly of fish, which is a very common characteristic of supplements.
Choose a reputable manufacturer. Check the supplement label or company website for mercury levels. Aim for a supplement with at least 300 mg of DHA and 220 mg EPA. Avoid supplements that taste or smell fishy.