I think you're missing a trick here, Andy. I suggest some research into a species of fish which is a) edible, preferably delicious; b) loves to eat all the other forms of life inhabiting your ballast tank; c) thrives in the dark. Stock your tank, and at intervals (to be determined by more research) have inboard fishing parties through the 'bung', terminating in a fish and chips supper. (I find myself recalling the occasion when Thomas the Tank Engine got fish in his side tanks.) Voila! - a clean yet productive water ballast tank, and queues of people wanting to crew for you.
Can I come?
Michael