Canned tuna is as durable as any food can be, with a shelf life measured in years. That's not the case with fresh tuna, which, like other seafood, is extremely perishable. It must be held under refrigeration at all times before it's cooked, and any leftovers must be returned to the refrigerator as soon as possible after cooking. For example, pan-seared ahi steaks should only be kept for two to three days after they're cooked.
Life in the Cold Lane
To ensure the best storage life for your tuna, keep it cold from start to finish. Buy your ahi just before leaving the store, instead of walking around with it in your cart, and keep it cold on the way home with ice or a freezer gel pack if you live in a warm climate. Transfer it to your refrigerator as soon as you get home, and cook it no later than the next day. Pan-seared ahi only takes about 20 minutes to cool after cooking, and it should be packaged and refrigerated immediately. Don't just brown the surfaces and then refrigerate the ahi for later cooking. The quick searing warms your tuna enough for food safety to become a concern, if it's returned to the refrigerator afterwards.