Betta Fish: What’s wrong with him :(

So I’m EXTREMELY new to owning fish. This Betta I got about two weeks ago. He was in a green green gross cup, and he was so happy when I transferred him. Tail grew and changed color, it was big and he was flourishing. I did my first water clean out, a 50/50 change (now that I’ve done research a 25% would’ve been better…) and he’s had curled tail ever since and doesn’t greet me anymore and get excited like he used to 🙁 I’m so sad and I think I hurt him and I just need help 🙁

He’s a half moon twin tail.

Ps: we are upgrading his tank and taking out fake plants and putting real ones, but if he’s sick I want him to heal before I introduce a ton of new stuff again. Please help!!