Is the ONLY symptom bloating? Just bloating seems odd. Really without any obvious symptoms you can't know for sure without some autopsies which require a microscope and some veterinary knowledge. Do you think you could cut one open maybe if one dies? We need to see what's swollen, the swim bladder, intestines, stomach etc. Do they have trouble swimming? When did this start to occur? Add any new fish or plants?
The first victims were a platy and some neon tetras
The platy suggests something parasitic. Common livebearers are almost ALWAYS disease ridden animals.
I want to say parasites but I can't be sure. Try feeding some peas to clear any intestinal blockage and see. Observe their poo. Its coloration and texture (extremely thin, pale etc.) can tell you a lot.
I can suggest you get some garlic and mash it up a bit to get lots of good garlic juice and soak the medicated foods in the garlic. It should make them take it a little better. Plus garlic is naturally antiparasitic so there's that bonus as well.