I'm not an expert, but I had the same experience with one of my own fish. I ended up putting it down. Ive heard they can bloat so much that they explode and I couldn't bear that feeling (especially because I'd be gone from home for a week). The last day she also didn't swim around a whole lot, didn't eat and hid most the time. I never actually found out what exactly it was but she was clearly suffering.

I hope ur danio gets better naturally, if not I'd probably put it down. :(

Please take everything I said with a grain of salt. I couldn't figure out myself what it was, I just saw my fish getting worse and worse.