if it's just regular bulk floss then theres nothing to worry about. my floval 304 does that when I fiddle with the media usually goes away in a day or so. What I've done to help it along is 2 things. First I buy poly pads they're made by coralife called Pure Flo and they come in 50 or 100 micron sizes and they work well you just cut to fit on top of the floss. The other thing is I've changed the orientation of media in the fluval. its actually backwards of what the manual says but works great for me and I know Freshmike who sets his up the same way
so in my media chambers from bottom to top is
floss, poly pad, carbon(optional),bio media(ceramic rings)