Scott:
< Looks like you've been talking with Bingman.
What levels to you keep said halogens at?>
Yes, I was alerted to the concerns about them from Craig.
I treat each a bit different:
Fluoride
Since fluoride isn't present in most salt mixes, I bring the salt mix up to NSW levels of F-. Then, I add a NSW amount of F- approximately every 3 months. Since Craig initially reported it to rapidly deplete, he hasn't said much. When I asked him a while ago, he simply replied that he was still working on the issue. I figure that once ever 3 months for something that depletes is quite conservative, and should never result in concentrations much above NSW.
Bromide
Since bromide isn't present in most salt mixes, I bring the salt mix up to NSW levels of Br-. Then, I add a NSW amount of Br- approximately every 12 months. I recall Craig saying that Br- didn't deplete very fast, but many organisms clearly do use it. I figure that once per year, even if it doesn't deplete is conservative, and shouldn't result in concentrations much above NSW (since I do water changes periodically).
Iodide
I add a NSW amount of I- (0.02 ppm) approximately 1-2 times per week as NaI. In my experience, I- is rapidly depleted (1-2 days), and this dosing regimen did not (and now should not, though I no longer routinely measure it) cause any buildup of I-.
Iron
I add iron exclusively for the growth of macroalgae. In the first year or so that my refugium was set up, it was eventually overrun with microalgae. Someone suggested dosing iron, and when I did so, the microalgae disappeared (presumably because the iron boosted the macro at the expense of the micro). I dose quite a lot of iron (II) citrate (way, way above NSW levels which are often limiting to algae growth). I took the initial dose from a Tullock book, but then altered it as I found appropriate. I've never noticed substantial microalgae growth to make me stop the iron dosing, but I wouldn't do it unless I had a macro or turf algae scrubbing system.