Description function piece #10 filter

[zapatower]

Hello MFK;
Which is the function and effect of the piece #10? Is a Venturi Piece? or maybe is a water jet to aerate the tank surface like a fountain cascade? Please help me.

 

[Rass]

My guess would be a surface skimmer

 

[deeda]

I agree it is a surface skimmer and from the pic it appears it will slide up and down on the vertical piece #11 depending on the water level in the tank.