Well I'm not sure which you are referring to because there are a lot of additional tanks. Normal fuel oil, collecting and expansion, clean fuel oil, lub oil, fresh water, battery water, sanitary, and a host of other miscellaneous tanks for hydraulic and vent, reserve hydraulic, mineral oil, my favorite torpedo alcohol, torpedo oil and compressor oil.
Are you referring to fresh water tanks? Seems most logical. Or engine oil sump tanks?