Your puppies have tapeworms. They cannot die by changing food or adding something in their food only. Give them deworming medicine. These worms can cause some serious disease in them. Your puppies are lethargic because worms are eating up all their energy. They do not let puppies consume their own food. These internal parasites feed on puppies' food. Try to get rid of them soon. Also read the label before giving them any kind of medicine. Its better if you take them to vet for an exact prescription according to their age.
