My fish's tail is damaged and it always floats to the top...?

Earlier this week, my sister's coworker moved and didn't want to take her 20 gallon fish tank with her, so I took it. There is an angelfish in there whose fins got tattered in the move, but it seems comfortable enough. The tank was pretty empty, so I moved my 7 danios into it. After a few days, the fish seem to be getting along just fine, but I've just noticed that one of the danio's tails is damaged, and it seems to involuntarily float upwards. It also tends to drift, still involuntarily, towards the filter, at which point it gets scared and rushes away to the bottom, and the cycle repeats. Eventually, it finds its way to the far corner and just sits there until something spooks it. A few things worry me- why is it floating? What can I do about it? I've put a small mesh thing in for raising fries that I got when one of the danios laid eggs (none of the males were mature enough to fertilize them, though) that it could swim into, but I didn't want to force it to stay in such a small space. I've also added lots of plants for cover, but it keeps floating, so it can't hide in them. Perhaps more importantly, why did this happen? Did its tail get torn because of the move? It isn't the smallest of the danios and it is only slightly smaller than the fish that were in the tank before. The angelfish is still only about three inches long. I thought it would be safe, but maybe it isn't? Should I move the danios back to their 10 gallon tank?