Neither will tell you precisely the deficiency, but both can be used to narrow it down, and combined with what you know you are putting in the tank, will help you fix it more easily by trial and error.
I'm not quite as against test kits as many on this forum, but I recognise them for the blunt instrument they are, and use them occasionally within those limitations. For example I wouldn't use an iron test kit to tell me exactly how much iron I have in the water column, but I'm happy to accept that it can tell me if I have a lot, a little, or none. You should be able to calculate exactly how much iron you are dosing, so that is your starting point. If your test kit is telling you that you have zero iron at some point in the week, and returns a non-zero result earlier in the week, then I think that enough to work initially on the basis of an iron availability issue to start with.
Breaking down the other information you've provided, you say:
but then say
Which is quite a lot to dose as others have pointed out, but also suggests if you are going from 0 - 'a lot' you are dosing all your micros on one day? So are you dosing micros daily or weekly?
If weekly, then as I'm guessing you live in a hard water area, it could well be as
@dw1305 points out:
The solution may be to try DTPA chelated iron that remains in the water column for longer in hard higher pH water, and to dose micros daily.
What other dry salts are you using, and what recipe? hat time of day do you dose?