I had a feeling. Just like yours, mine suddenly looked and acted strange. He got looking kinda scragly and was acting lethargic. In a few days though, he got much better. In fact, now he's only got a slight hump and is almost like he was before. Although he still tends to keep away from the other two danios (he hides on the bottom in a decoration.) This happened with two different zebra danios. The first time was a few months ago and that fish lived for another 2 months.
However, I've been experiencing many guppy deaths in the same tank. This morning, the last died and I was looking at it in sunlight and I saw a 1/8 inch worm swimming around in there. So, I'm pretty sure that they are the culprit causing havoc in my tank.
I was asking on another board and her danios took on the same shape when they were ill.
So, like jgulecas says, I suspect that there is SOMETHING gone wrong in there. Maybe not disease, could be parasites, or possibly even stress. I don't know if it's a fact, but now, I've heard from a couple of sources of a dramatic change like this in their danios when under attack from something. You indeed may want to quarantine the fish, or at least, minimally keep a very good eye on it and the other fishes (what else do you have in the tank?)
