Well, I think that one of my very dear fish is on the verge of death.  She has been swimming vertically (head down, tail up) for hours with an occasional mad, twirling dash around the tank for three days.  It seems like she does not have any control over her tail/lower half because she is consistently floating upwards.  We thought the worst multiple times today, but then she freaked out and swam around (it sort of looks drunken).  I have no idea what to do!  I do not want her to suffer, but I cannot 'put her out of her misery' either.  I do not know much about fish diseases/ailments, so any ideas would be helpful.  I think that it might just be old age because I raised her from a baby (swordtails are live breeders) when I was in fourth grade and now I am a junior in college.  Weird antics aside, her body seems to be healthy but she is not eating.  ???