I agree that I would stop the electrolytes and egg, and concentrate of getting it to eat chick starter as it’s main diet. Electrolytes can cause diarrhea if used more than 5 days, and are not necessary. Plain water is best. Egg is fine in small amounts as a treat later, but not required. I worry that your chick might have possible coccidiosis, so it would be good to get some droppings checked with a fecal float by your vet. Corid in the water is the treatment for coccidiosis.
Mixing new chicks in with it, could be a set up for coccidiosis, so you can go ahead and treat for it, just in case. Dosage is 2 tsp of the liquid Corid, or 1.5 tsp of the powder for each gallon of water for 5-7 days. Do you know for sure that your earlier deaths were due to brooder pneumonia (aspergillosis) or respiratory disease? What were the symptoms? Hopefully this chick doesn’t have some as well.
Mixing new chicks in with it, could be a set up for coccidiosis, so you can go ahead and treat for it, just in case. Dosage is 2 tsp of the liquid Corid, or 1.5 tsp of the powder for each gallon of water for 5-7 days. Do you know for sure that your earlier deaths were due to brooder pneumonia (aspergillosis) or respiratory disease? What were the symptoms? Hopefully this chick doesn’t have some as well.