currently its 8.3, but its been as low as 7 ( was away and water lvl in sump dropped ) and as high as 9.8 ( when I shut off return to do stuff in tank ) and it just keeps doing its thing.
Specs say 7-9 is optimal. So I guess in theory its true-lol

Thought I would do an update

Had this a year and a half now and still happy with it.
My bioload is much bigger now with a large atlantic blue tang and a 6 inch emperor and misc other fish and this still skims like a champ.
Done a full strip down a few times to clean the pump etc other than 1 of the plastic screws snapping its been pretty easy to do.
Still runs quietly and does a bang up job.