Ghengis Kahn

Our first pet is dying.
His name is Ghangis Kahn, and he is a Betta Fish.
He hasn't eaten for days, and the fish man at PetCo said that he is probably just old. He advised me to get some Fresh Water Aquarium Salt, which is like a fish Gatorade. He said it probably won't help. Poor Ghengis needs something.
I am sick to death of death.