Hello,
The change in food may have caused an allergic reaction for your gerbil. Hartz food is not one of the better quality foods on the market. I would start the Gerbil on some Plain Pedilayte(electrolyte replacer for infants) via eye dropper. I would also get some Oxbow Critical Care and start to feed the Gerbil. This is a natural food that is for sick herbivores. Once the Gerbil seems better you can get back to a quaity gerbil diet that you can get a a Pet supply or build yourself. This should help. with diet and care:
http://agsgerbils.org/Learn/Gerbil_Care_Handbook/index.php
If the gerbil does not seem to get any better, then a Pocket Pet Vet with gerbil experience may be needed to treat appropriately. Joan
Veterinary Technician
I am Vet Tech. for 30+yrs. who works with all animals. I do alot of rescue work with small animals.