Seems like you were down the coal hole for a wee while on that very tidy installation. I started using some of that blue coloured additive when I installed the holding tank on our Carpenter, it seems to stop the horrible smells. I'll fess up and admit I don't know what its environmental credentials are like, just add a capful when flushing a no 2.
For my sins I replaced two magnificent but frozen Bronze Sea Cocks with Nylon/glass units and replumbed the cockpit drains on the Hood 38 and the next job was to replace the steering cable, which was fun but at least I know how it works now. Bei