As you’ve learned, having “popeye” is like having a “cough.” The symptom gives almost no clue as to the cause.
What’s a fluorite substrate? any chance that your bottom dwelling fish are getting sick because there are some issues peculiar to the bottom of the tank?
Get a syphon that removes detritus from the substrate and give it a good going over. Also, maybe try a course of antibiotic FOOD. Not drugs in the water, but food in the fish. Follow the directions and don’t cut the treatment short, just like antibiotics you’d take yourself.
Finally, stop buying from PetBox. Find an independent store with knowledgeable sales people. They’re more likely to have healthy fish, and more likely to be able to help you when you have problems.