To piggy back on Hans' comments, I agree that Mn to me represents a real limitation of the "dose and test" approach. Much moreso than most other elements.
After watching ICP-MS on my system for months, sometimes with no Mn input, sometimes with a trace supplement (unknown Mn level), sometimes with water changes (known several ppb/week Mn input) - Mn was always depleted down to nearly the LOD of ICP-MS <0.2ppb.
Also observationally, during this time - I note more microalgae flushes of brown/green on the glass when Mn is being input vs not. That along with other things I've observed makes me think Mn is likely acting as a limiter for some photosynthetic growth.
So - yes my Mn is low. Whether I dose none, a little, or a lot - it's still essentially within plausible measurement error of zero. If I was to be determined and added so much Mn that I actually started to accumulate it to easily measureable levels somehow, then presumably exponentially growing microalgae (diatoms, etc) would multiply until they've depleted something else. That doesn't seem desirable either.
I'm not sure Natural Seawater levels as a guide is helpful in this context either. If I can add 1x or 10x or 30x NSW levels to the tank weekly and it stays just as depleted as it was with no additions, then I don't see how NSW tells me anything about how much should be going in.
Posting this because I don't have good answers. My best guess at this point is that for Mn a "target level" isn't a great idea, a target input rate (ppb/week or day) might be a more sensible approach. I have no idea what that target input rate should be, other than I can use the fact that a 10-15% water change per week on my system with my salt is adding 3-5ppb Mn per week, and that doesn't seem detrimental, and I see organism response. And Hans' point - observe the tank carefully has to be part of it.
