Supplementing with vitamins, minerals, electrolytes & probiotics is often done with poultry to improve feed conversion, increase egg quality, fertility, and prevent disease. Many factors can cause loss of weight and anemia. Internal and external parasites are often the culprit if birds are on a well formulated ration. This can be prevented but when symptoms appear, there is no time to waste. Check the bird over it's body by parting the feathers and looking for mites or lice. If no external parasites are seen, look down the throat, check the eyes for clarity, and check the droppings with a bright flashlight for anything wiggling around within them, diarrhea, blood, etc. Internal parasites like protozoa and worms, pathogens, can all cause enteritis which can cause irreversible intestinal damage. You can take the bird to a vet and get a fecal sample examined, or try to treat the bird based upon symptoms.
I would treat for coccidiosis, worm the birds, then supplement with vitamins, minerals, and probiotics in the water.
