Very interested in this discussion! I dose BRS 2 part...Alk and Calcium and rely on monthly water changes (hW-Wiegandt Marinemix Reefer) for trace element replacement. Very curious as to what folks use for trace element and/or iodine replenishment.
Having tried the Dutch Synthetic Reefing system (yes, it's been around for about 15 years and was named something else before other's marketed it as their own), I just didn't see it worth the time and effort. Does it work? Sure! 100%, same as most methods if executed correctly. I just couldn't justify the money and time to do it correctly versus just changing some water fairly regularly. I've used daily AWC (about 2-3% per day) and over the past few years, and my trace elements are always on point. Why try to dial everything in when you can just hit the reset button with a water change and not worry about over or under dosing? Mind you, I have two systems and while one is using a calcium reactor, the other has always been supplemented with 2-part, and both system's trace elements have always been solid.
Plus, I'm sure I'm ignorant on the subject, but I'm not convinced of how vital some of these trace elements are. If they exist in absurdly low concentrations to begin with, then how can they be that vital to coral growth? For instance, look at Vanadium. It exists in the ocean at .00153ppm (source: Randy Holmes-Farley). Carbon monoxide exists in the atmosphere at .09ppm, which is about 59x more concentrated in the air we breath, than the water that corals thrive in, and it kills animals. There is no beneficial side to carbon monoxide as far as animals are concerned, and to add more into the atmosphere would be insane. And yes, at any significant levels, Vanadium is toxic to ocean life, just the same as carbon monoxide is to land animals.
In addition, if the concentration, using Vanadium for example again, is .00153ppm then there simply isn't much to be used, even if corals could. Not to mention that replenishing it would take so meniscal of an amount that any dosing solution would basically just be a bottle of RO water. Haven't tested it yet, but my tap water might have more Vanadium in it than any potential dosing solution has.