How much Dupla fertilizer to use?

I've been using DuplaPlant tablets and drops for several months now,
but haven't had decent results at all.  But I've only just started
doing CO2 injection (in a totally new tank.)  In this totally new
tank I've got CO2 injection, undergravel heating cables (DIY, 20 watts)
in and it's 45 gal.  Oh and it's also got Duplarit in the substrate
(again, all done according to the directions on the box.)

According to the box (or what I can make of Dupla's attempt at English),
I'm supposed to put in 1 tablet per 50l of water, my tank is about 200l,
so that's 4 tablets.

And I've heard alot of people say that the drops are necessary, but only
needed in very small doses - I believe George B. said that he only uses
2-3 drops/day in his planted 75's.

But even after putting in a double dose of both I'm barely getting any
hint of a reading out of my Red Sea fish pHarm ltd. iron test kit.
From what I can get out of reading the Optimum Aquarium, I should have
iron in the .2 - .3 ppm range.

Is this right?  I thought perhaps my test kit was faulty, but I found that
if I crushed some of the tablets or put some diluted drops directly into
the tube I could get positive readings.

About the only clue I've got is that this, the second bottle of the drops
that I've purchased, is *definitely* not as darkly colored as my first...
Does anybody have any idea what the deal is here?  And what levels of
iron do I want to maintain my water at?

                                                - Steve
Steve Benz (steveb at realtime_net)   |   Pragma Systems, Inc.
Author of ZipPiz, a Windows utility for unpacking downloaded archives.
http://www.ccsi.com/~pragma/zippiz.html | ftp://ftp.ccsi.com/pub/pragma