if there is no smell to it (caused by water getting into the container- sort of a nutty smell), then it shouldn't poison your fish, but it may be very low in nutrients, so wont be good for a staple food.
if you have lots of other foods, and just want to use it to avoid wasting it (but can still provide other high nutrient food) you should be ok, but then again, if you have lots of other foods, why bother using a low value one?
if in doubt, chuck it out.