User blog comment:Xilinoc/Ch. 600 - SNIPE/@comment-5693502-20141008195917/@comment-24913480-20141008202154

The liquid in the tank might serve to preserve his zanpakuto, to prevent it from rusting or damaging over time (I'm guessing that sword, and Nimaiya for that matter, is extremely old - some 6000+ years). And also to make sure his awesome doesn't break the world in half.