Betta Fish: Clamped fins

Hey so I’ve had my betta for a couple weeks and he was nice and flowy, he did glass surf but stopped after a couple weeks and figured everything was fine as he ate well swam to top for food and to say hi but then this Friday I went to do a water change and he’s sitting on the floor with clamped fins:( I don’t have a water test kit as this hasn’t happened before(will buy one now tho) he still swims sometimes and can stay afloat sometimes his back will sink he doesn’t seem to be coneing, he still eats and will go to surface but his fins are clamped and he lays on the floor in the back of his tank. He has a 5+ tank with places to hide and his heater stays at 77ish I bumped it to 80 tho. When he sinks if I tap or look at him he’ll go and swim around then go lay back down:( (((WATER is crazy cause I took photos after a 10% water change, but if there’s anything different I should do to tank pls lemme know))))🖤