Don't mean to resurrect an old post (or do I?), but actually, there is a way to test for missing elements... and NEVER have to do a water change. It's called the Triton method (Triton-US). Water is tested for precise concentrations of lesser known elements found in seawater that are either essential and missing, or detrimental and building up to high levels in the aquarium. The elements being tested for include Na, Ca, Mg, K, Sr, B, Br, S, Li, Be, Ba, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Al, Si, As, Sb, Sn, Cd, Se, Mo, Hg, P (PO4), Pb, I. It takes a few samples ($49 a pop) to get things balanced, but once you hit the sweet spot, you'll have the most stable system possible. As you know, even simple water changes alters the tank's parameters... reducing stability. While it's a bit pricey, if you've spent thousands on your system and inhabitants, what's a few hundred more? LOL!