Feed it to live food cultures and get fresh stuff. Vitamin C especially likes to break down quickly, no matter how stable the manufacturer claims it is. Also keep in mind that it's spent some time on the warehouse and then store shelf before you got it, and now that it's been opened, being kept near a source of humidity (the fish tank) is not going to be good for retaining nutrients.
If you have people in the area that you can split food with, that's a good way to use up fish food in a reasonable amount of time. Also lets you buy in bulk if there are enough mouths to feed.