Thinking Kalk/top off

[Delphinus]

Kinda wondered the same thing but then realized that it's a powerhead that's supposed to sit in the freshwater reservoir. So it's the pump that feeds the water into the mixer.

Am I close?

It looks like a minijet. Please explain what sort of pieces you used to make the cap for the output? Is it completely watertight? I'm particularly curious how you made the piece the tubing slips into.