Gerbils over 18 months of age will sometimes develop heart failure.
The symptoms of heart failure are a lack of energy, labored breathing, and a clicking noise when breathing. The clicking noise is just like that of a gerbil with a respiratory infection, and he may, in fact, have even developed pneumonia due to his heart failure. If it has reached the point of pneumonia, he will probably not have much time left, and euthanasia should be considered.
If heart failure is suspected, your veterinarian may want to confirm it with an x-ray.
There are medications that can be tried to give you a little more time with him, if you wish to try them. They will not cure him. It may even be too late for them to do any good. The medications will have to be given every 8, 12 or 24 hours, according to the vet’s instructions, for the rest of your gerbil’s life. I have had good luck using a combination of furosemide, which is a diuretic also known as Lasix, and a heart medication called enalapril, for three gerbils.