it sounds very much like dropsy. It is kind of an odd disease, it is not a disease that is epidemic in your tank. Some people think it is from bad diet, but even some fish that have had a diet of high quality life food occationally suffer from it. I really don't think it was singliy from the high nitrates, but could be a factor. There is no real cut and fast treatment for this disease, but I did find this in my travels as a dropsy treatment:
Put the fish in a hospital tank, but do not add any salt. because it irritates under the fish's scales. Put the temp fairly high to about 78 up to mid 80s. Put in epsom salts 1/8 teaspoon/gallon. You can try one treatment of oxolinic acid can help. But you need to use caution becase the oxolinic acid will become toxic in a few days (about three) and a complete change of the water needs to be done at this point to get the fish out of the presence of the acid.( remember that the water has to be preveously cycled water)Then, if the fish lives, it should be left in the hospital tank for about one week. This was given to me by a friend who has been in the hobby for over 50 years, I have never had to use it, but I hope it works for you. I have no Idea what oxolinic acid is, so that is up to you to find out. Also it can be benificial to feed the fish antibiotic food (which your LFS may have). You are going to have to work fast though, because the majority of fish with dropsy die within a week or two!! We can only pray that you fish will survive this! Good luck!
It sucks, I know. I lost my favorite betta ever to it in January - for no discernible reason.