It doesnt only happen in new tanks, but it is more likely to happen in new tanks. When the tank is cycling (which I assume yours is) the water is toxic, and can cause fish all sorts of problems.
Well as many who go into this do, i went to the lfs and asked them what to do. From there advice i thought leaving my tank a few days was good enough. However i only had one fish in it for the first week so the other fish were put into a week and a half cycled tank, which i know prob isn't much better.