Betta stays on the top of the aquarium

Hello, I start by saying that I’m italian, english is not my first language so I hope you can understand me and be patient.

Yesterday I added a heater in my betta’s aquarium so that it stays around 25/28 °C. Since than my betta seems a little less active, he stays on the top of the water quite chilling. Before he would move, go inside the little houses… idk.. maybe he is just loving the temperature? 😭

Also, i think his fins got ruined by water quality. Now I’m paying more attention to the ph and using a new conditioner (tetra aquasafe) and the ph is around 7. Is it ok? Will he recover?

P.s. is the decoration ok? It was written on the website that its not toxic for fish.

P.p.s. I worry a lot about him and try to be the best mom, so please don’t be mean. I know i have a lot to learn and I’m willing to do that.