This is a basic marinade using achiote for fish. In order to bring out the best flavor, you must first combine the achiote paste with a sour liquid, like vinegar or citrus juice. Then you can balance it out with olive oil, and add flavor like garlic and oregano. Asian ingredients like soy and fish sauce also work really well in this type of marinade.

For about 6 filets of fish you will need:

1/4 cup achiote paste
2 oranges, juiced (combinations of orange, grapefruit, and lime all work well)
1/4 onion, sliced in thin strips
salt and pepper

Place the fish filets in a container wide enough to hold them. Whisk together the marinade ingredients, and pour over the fish. Marinate for at least 1 1/2- 8 hours.