film at top of water

[Marlin65]

Not enough flow if it is your main tank. Add some power heads and they should take care of that.

[reefmutt]

that film usually indicates an excess of nutrients in the water...It could be protein buildup or a fine algae layer.
adding some carbon and/or phosphate remover, and/or protein skimming as well as doing water changes will also help get rid of it.

[Snaz]

Reduce the height of the overflow weirs so the water flows out truly from the surface and not from just under the surface.