Ocean Free ultra slim hang on filter with skimmer

[rainbatt]

filter.JPG

Anyone knows how does the skimmer work?
Used it for a month but saw no bubbles or waste, maybe setup wrongly.

[Urban Aquaria]

Its a freshwater surface skimmer (not a saltwater protein skimmer)... its job is to just draw in floating debris and oil film from the water surface, help to keep the water surface clear.