Buying regular pet food isn't exactly gifts for the pets. On the other hand, buying special treats can certainly be called gifts. Special treats, special toys, are probably the only things the pets want, as well as what most pet owners would buy for their pets as presents.
Me too stocked up frozen food for my fish as I see it as a chance (holiday season on sale, free shipping, as well as the freezing temperature can keep the packages nice and frozen on the way). Since frozen food for fish are weekly treats, they have got their Christmas gifts as far as I am concerned. Other than frozen fish food, I got a new extra large fake tree log as decoration. Some fish might like hide and seek. Extra hiding places in the tank can be wonderful for them. Now I call that another Christmas gift.