The normal usage of expirey and best before dates in the food industry is that:
Expirey. product should not be used after this date as it might not be good for you. (just like what Ira said)
Best Before: product will be still good up to the date stated but can be consumed after that date without much detriment to your health but your taste buds might rebel. ie Bread, it has a best before date of a week but you can still eat it after that provided you don't mind eating mould
but with fish food..... you can take a risk.... if it has bugs crawling all over it then you have live food which is better than flake