IMHO, I wouldnt feed frozen seafood older than 3 months unless you can make the package air tight. Not saying it goes bad being in there longer than 3months, but seafood looses its nutrition faster than other frozen meats/veg. If you plan on keeping it longer than that, I would recommend soaking the food in something such as selcon and other vitamins regarless if its been added to the food prior to being frozen so the fish are getting some form of nutritional uptake from the food instead of it just taking up space in their stomach.