I know I'm (dangerously!) assuming here, but I'm guessing this is a new tank, and not cycled at all.
99% of fish 'diseases' are caused by poor water quality, so that's probably what's affecting the danios too.
Do a 80 or 90% water change, every day, with temperature matched, dechlorinated water, until you can get the water tested; preferably with kits of your own, but if you get your LFS (local fish shop) to do it for you, it's most important that you get them to write down the actual numbers; all too often they will say "it's fine", or "a little high" and that's of no use to us at all!.