Fish oil is the most bioavailable source of preformed EPA and DHA omega-3 fatty acids. Common sources: salmon oil, menhaden oil, sardine oil, anchovy oil, herring oil. Generic "fish oil" without species naming follows the same quality logic as protein ingredients — named is better. Fish oil is highly prone to oxidation and must be preserved (typically with mixed tocopherols or rosemary extract). Plant-based omega-3 alternatives (flaxseed, chia) provide ALA, which dogs convert to EPA/DHA inefficiently and cats barely convert at all — fish oil is meaningfully better for both species.