I brewed my first sour beer about a month ago, and it has been in secondary for about two weeks. I purposely mashed high so the SG after 3 weeks was around 1.016. My main concern is that the beer level goes just up to where the carboy starts to taper in, probably about 3/4 of a gallon from being mostly full.
I'm not too worried about oxidation in the short term because the bugs still had plenty of sugar left, and I haven't opened it up since racking. I plan on adding some oak cubes in a month or so, but I'm not sure if I should bother topping it off with beer or water.
The beer is a pale Belgian ale with dregs from a brett beer, Jolly Pumpkin, and two lambics pitched in the primary. My goal is for it to be fairly balanced between brett flavors and sourness, without any strong acetic flavors. I know different yeast and bacteria thrive in different settings, which makes me think I might want to do something about the head space.