Aoutch. Does not sound too good for Mr. Betta. If you are sure your
water is not in question, then one of two things could be at play.
Either parasites or bacterial infection. If you shine a flashlight
on him do you see either white spots (ich) or a gold like all over
very very fine powder (velvet)? if so, treat with maracide or any
other velvet medication. A water change and sanitization of his home
first is a must. In my opinion you will not see either because I
think you are dealing with a bacterial infection. You mentioned
antibiotics but did not mention which. I would go with Spectrogram
or Kanacyn, double dose. Have you fed live foods? If so, your supply
may have been infected. I think your betta only has a 20% chance of
recovery, but don't give up! Do double check your water, make a full
water change (very gently) and also make sure there isn't anything
in the tank that could have contaminated the water (any metallic
object, sea shells or coral, any harmful household cleaning
chemicals, soap etc...). Goodluck with Mr. Sickly Betta.