Ethoxyquin is a synthetic antioxidant developed by Monsanto in the 1950s and used heavily in pet food and fish meal preservation. Concerns about possible carcinogenicity led to FDA voluntary dose reduction in 1997 and outright ban in EU pet food in 2017. It's still permitted in US pet food but rarely used in premium brands. The biggest gotcha: ethoxyquin can be added to fish meal at the supplier level without appearing on the pet food label.