Ditto: Big water change to remove ammonia and other things from the water. Clean the filter, taking it apart and lightly scrubbing the impeller and anywhere else the fish contacted.
I am not sure if any symptoms you are seeing are from the sudden rise in ammonia from the dead fish, though. That does not happen the instant they die, but builds up over several hours to a day.
Looking sort of swollen is not a common reaction to ammonia. Ammonia burns the gills, so the fish have a hard time breathing. They will usually swim close to the surface and take in air through their mouths. Lethargy, hanging out at the bottom could be related.