Right first of I've not used a protein skimmer in my tanks for over 17 years as you all know ,
Sometimes you can get a slight film on the surface of the water , looks a bit cloudy ,
Juwel ecoskimmer is the bollox for curing that and only cost around £35 .
Had a little problem like this myself recently, could be where gobie sifts the bed and is creating a little bit of a film , but anyway the film or cloudiness that forms can effect lighting getting through and oxygenation ,
Anyway I got one , put it in and wow what a pukka little surface skimmer . Well impressed.