Green Film

[Silverlady]

Hi everyone, I haven't been on here for a while, my problem is a green film on top of the water in my 4ft tank.
The water is clear and the fish are happy. Is this algae or somthing else, it clears when i do a water change the comes back.

 

[Curiosity101]

I'm not sure what it could be, could you maybe get a picture?

But it may just be that you need to disturb the surface a bit more, what is your surface agitation like?
Have you tried lifting the filter outlet up a bit to get a bit more movement around the tank?