Danio, can't swim well

[Tempestuousfury]

What's up with my danio? It's somewhat bent, though not enough to be a real problem. It seems emaciated though I feed the fish frequently. I've only had it for a few weeks, and all of my fish have been fine.

It swims in surges, I can't see any dorsal fin (clamped or not there, not really sure...), and it stays in one spot. I've got it in a bucket with melafix and salt. It seemed to be swimming better this morning.

Any suggestions or possible causes?