Should the filter completely fill with water

I have a RENA XP 2 which I purchased as part of a new tank set up. I have used external filters before mostly Eheim one and so far have been pleased with the RENA, however it is not completely full of water is this correct?

A: Completely full is relative but I would say if you have more 1/2 inch of air in the top eventually you will start having problems. This usually indicates poor priming on setup or restart. If the air pocket increases in size after a few days then you probably have some air leaking into the system from somewhere.  You can find other Q&A's on this subject here:

http://www.planetrena.com/rena-filstar-performance-questions.html

http://www.planetrena.com/Filstar-Rena-FAQ.html

You may have to skim other questions some that relate to your particular situation.