If you want to get serious about trace element replenishment, you have to do constant testing and be ready to add supplements, if required. It depends on your livestock, of course. Even relatively frequent 50% water changes will not keep up with calcium needs of a high volume SPS/clam/halimeda macroalgae tank. (and water changes of that size are hard on your inhabitants, too) You'll need some sort of reactor or constant B-ionic type additions. You'll never keep up with Iodine, either. It gets consumed too fast.
If you want to keep on top of an excess nutrient problem, slow down on feedings, don't rely on water changes.
I think that water changes are more of a feel-good thing for us than anything........and don't get me wrong - I feel better too when I do one....
I would classify water changes as a bit of a "cushion" at best for any over-dosing or over feeding that we may tend to do.
But observation of the health of your corals and fish, water testing and appropriately adjusting the feeding/trace element supplementation is the best way, IMO, to keep your water quality at it's highest.
Here's some fun reading and charts, if you're interested.
http://www.reefs.org/library/article/tb_wctext.html
http://www.reefs.org/library/article...ll_wcdata.html
Mitch