The symptom you state IMO/E can be several things from nutrition, pathogen, swim bladder......
Vit C and D lacking in the diet can cause this
Bacterial issues related to fish TB, resp infection can cause this
Fallers as a fry and deformed swim bladder
Constipation or infection in the duct
One thing I have found to work for most of these issues- adding the infant multi vit to their food and/or 1-2drop/gal, epsom salt 1tsp/gal to reduce edema and a laxative effects
Water changes and lots of them, lowering the water level and turning off the filter
Heat depending on the cause, warm up or cool down, bacteria grows best in warm water but sometimes they need the water warmer to make them more comfy and to speed metabolism
Covering the tank top with a plastic veggie wrap to warm the air and make it more humid, when they breath too cold of air they can sometimes get resp infection and/or the duct can become inflamed.
Age related issues can also be a factor