kbauer,
Well, FW food is not a good choice for saltwater critters (different lipids). Also, depending upon the size of your anemone, a full fish may well be too large to be digested well. I feed my BTAs either a single defrosted krill, or part of a krill, depending upon the size of the anemone, about once every couple of weeks (my anemones also get quite a bit of processed food (Formula 1, 2, Prime Reef, Mysis Shrimp, etc), as I feed the fish a cube daily, and the anemones intercept a great deal of this food.
You say your anemone is "belly up." If this means the tentacles are still oriented upwards, this may not be a problem. If, however, the anemone is face down (tentacles in the substrate), this is not a good thing, as anemones respire through their tentacles and oral disk. Anemones deflate down to almost nothing periodically to expel wastes, so this may be what is happening to your anemone. Don't assume it's dead unti it actually begins to fall apart.
HTH,
Kevin