Bettas fins too heavy?

[WiccaFish]

I'm hoping someone can shed some light on what's wrong with my son's betta. He was rescued from PetSmart 4 months ago and was a bit shy at first but came around after about a week. He ate, swam, pooped, etc like any healthy Betta but about three days ago he took a turn and I have no clue why. He rarely eats (maybe one or two bugbites), hangs on the heater knob or on the bottom of the tank all day long now. I can't see any visual signs of infection or bloat. I'm at a loss. When he swims it looks like his back fin is dragging him down but they really arent that big. Water parameters are normal, temp is 79.8, live plants and I use a sponge filter. Nothing new was added or done. Please disregard the the mark on his head. He came that way from the store

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[PorshaF]

Do you know your water parameters? No2, No3, nh... is he a butterfly or rose tail? I can't remember which one it is but I heard a lot of fish having issues because of the weight of their fins. If it is that there isn't much to do besides reduce filter flow (you said you have a sponge so shouldn't worry about this one,) add floating plants so he can rest closer to.the surface or a betta leaf even will work. If he seems bloated try giving him a piece of a shelled boiled pea, that's a natural laxative and helps with swimbladder. Make sure to continue regularly changing the water once a week and I hope he improves!