Unless you're a vegetarian, one of the staples of your 21 day UltraClear detox will be fish. The fish recommended on the modified elimination diet are: halibut, cod, tuna, salmon and mackerel. Fish recommendations are always controversial because of concerns with both toxicity and depletion. Toxicity also varies based on where you are located regionally.
Seafood Watch is a great place to go for information and they even have a handy iPhone and Android app. While on the detox and beyond, making choices about what we eat is complicated and always involves striking a balance between what is available, what is harmful and what is beneficial. Nutrition is never an all or nothing game.