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.
Is It Normal For Puppies To Sleep A Lot & Not Be Interested In Food If They Have Worms? Or Could It Be Something Else?
Well it is a common thing for puppies to sleep a lot. They usually sleep for more than 12 hours in a day. But the loss of appetite is not a normal sign. Its due to the presence of those white worms in their digestive and excretory tracts. Parasites are responsible for making your puppies stay away from the food. You can rid them of the parasites by proper medication. I would suggest you to take them to a vet soon.