I'm having the same problem as well and cannot figure out how to get rid of it without getting a surface skimmer attachment for my Fluval 304. It looks like an oil slick or something kinda hazy, and when I put food in you can see the segments of the oil on the top from the regular water. I hope that makes sense. I have tried using purigen, then tried charcoal. Now i'm increasing my water changes. I have also tried to cut down on feeding but my Clown Loaches are huge along with my Rainbows and they eat everything I drop in before it can sink halfway down.
Ryan