I was thinking somethng like homeostasis (??), but without a mechinism to allow it...

This thing pretty much disolves away to nothing, right? Seems like you're already noting some good out of it if it's keeping you above 400. I haven't checked my calcium in a long time, but I noticed now with the water changes and the additional scoop of kalkwasser I'm getting a lot of new growth from my SPS.