I found this excerpt in the link below ... may help.
Rena Filstar is not sucking water
the water suction is not very powerful it works but not to full potential does anyone have advice
A: This symptom has to do with priming and has been discussed already. Please review previous posts at the bottom of the Filstar FAQ page for suggestions. I also advise viewing the video on startup at MARS FISHCARE :: WELCOME TO MARS FISHCARE
Rena Filstar Performance Questions
75g Planted Barbs Denison, BlackRuby, Tiger, Albino Tiger, Green Tiger, Cherry, Gold, Checkered, Odessa.
NonBarbs Cardinal, Neon, RummyNose, Bloodfin Tetras, Z. Danios. 1RTS. Amano, Bamboo, Ghost & RCS. 20g Long Shrimp Tank RCS, Tiger, Amano.