Clamped fins out of nowhere

[Enlistee]

My betta has clamped fins and is just lethargically floating at the top of his tank, he is eating whenever i put food in and is otherwise normal. This happened the day after i changed his water that day he was happily swimming around completely normal and over night has changed. What can i do to fix this?

[RedBetta]

How big is your tank? Do you have a filter?Heater? Do you use water conditioner? How often do you do water changes? and finally, do you do 25%,50%, or 100% water change when you do the water changes?

[Enlistee]

2.5 gallons , 25 watt heater , yes when needed i use water conditioner although i have been using filtered water. water changes 2-3 times a week and they are 100% changes

[ScuitoAmi150]

He might be cold, they all tend to clamp their fins when they are cold.

[Enlistee]

ok thanks ill try puting up the heat a bit

[Enlistee]

today he is a bit more active he still just sits there most of the time but is actually swimming around following my finger around the tank, fins are still clamped though any ideas?