Hi. I would like to first say that there are no true swim bladder meds because the problem that causes swim bladder disorder is constipation. All that can treat that is peas or starving them for a few days.
Your problem sounds like one of two things... swim bladder disorder or an internal parasite. Are there feces trailing? I would recommend feeding peas first... perhaps your fish has a minor plumbing back up, but not enough to affect the swim bladder. Try the peas for a few days, and if the swelling doesn't go down let me know and I will let you know what the next step is.
I would hold off on the upgrade to the bigger tank until results show on your sick one. I would say for right now your fish can stay in the main tank, but you may want to set up a hospital tank with 1/3-1/2 of your main tank water and the rest spring water for if you need to treat for parasites.
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Phishie

"An ounce of prevention is worth a pound of cure."