WordNet records no synonym and no antonym for tankage. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
the quantity contained in (or the capacity of) a tank or tanks
the charge for storing something in tanks
the act of storing in tanks
Synonyms, antonyms and the broader and narrower relations for tankage come from Princeton University’s WordNet. Full sources and licences.