Kinda sounds like what happened with my first betta, who died a few weeks ago. i mean it might not be the same at all, but at first Swish was lethargic, hid a lot, rested a lot, still ate. he started showing symptoms of disease and he got progressively worse, although he continued to swim for food until his last few days, even though he jad troubles swimming.
if i were you, i would try putting him in a smaller 1 to 3 gallon quarantine tank, with 1tsp of epsom salts per gallon, and fast him for a week. Change the water every day to every other day. if he doesn't seem to be getting better, try bumping it up to 2 tsp epsom salt/gallon.
Also I've heard in a few places on the forum that feeding peas to bettas isn't a great idea for constipation.
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