Marinating ahi tuna -- or any other kind of fish -- is not strictly necessary as it is for tougher cuts of meat. Unlike beef, fish is naturally tender and gets less so as it is cooked, which is why overcooked fish is dry and tough. It is possible to marinate ahi tuna to add flavor, but you have to be careful not to soak it for too long. A good ahi tuna marinade will consist of an oil, a small proportion of an acid like citrus juice, soy sauce or wine and some herbs and spices for flavoring.
Rinse your ahi tuna steaks under cool running water, and pat them dry with paper towels.
Pour a half cup or so of olive oil into a small bowl. Add about half that much pineapple juice. The exact amounts will depend on how many ahi tuna steaks you are marinating. You want enough liquid to cover them completely.
Add a squeeze of lime juice and taste the marinade. Adjust the proportions of the ingredients to your taste.
Stir in a sprinkle each of cinnamon, cloves, garlic salt and chili powder. Taste the marinade and adjust the seasonings.
Place the ahi tuna steaks in a plastic zipper bag large enough to hold them and close without tearing.
Add the marinade and massage the bag to cover the ahi tuna steaks entirely. Close the bag tightly and place the bag in the refrigerator for two hours. Remove the ahi steaks, let the excess marinade drain off and cook them however you prefer.
Experiment with different oils, acids and spices to create your own unique marinade.
Never marinate fish fillets for longer than two hours or the acid in the marinade will begin to cook them.