Your overloading the bettas system with too many solutions too quickly without know what the problem is.
if your water quality in the main tank was bad that could have been fixed with simple water changes.
I would suggest you do a 50% water change on the bettas current living quarters. Don’t dose anything new. And monitor the situation. Do daily water changes and see if he improves?
while you wait for results of that water change and watching your bettas for changes go take the parameters of his original home and post the results here so we can at least see what kind of water he came out of.
then do a 50% water change on the tank he came out of. Wait 24 hours, re-test the water and post those parameters here.
while you have your test kit out, you might as well test the water the betta is currently living in as well.
if there’s no outward signs of skin damage, something on the skin, or if he is still eating them there’s nothing to do except clean water and monitor for other signs of improvement or getting worse