If it's food you'd eat, it's fine for the fish. The FDA pretty strictly regulates food sold for human consuption. Now if you let it sit in your fridge for too many days before preparing the food, your fish face the same risk you would. (It's usually not the bacteria that gets you so much as the toxins they have been producing on the food. The toxins lower your immune system, leaving you open to the bacterium that is usually harmless to you.) Most homemade food recipies I've seen call for making up the food in large batches and either freezing it in small portions or baking it and crumbling into flake. These will preserve the food in the condition you bought it. Many recipies offer suggestions for storage of the food, and expected expiration dates.
