Iron levels

As some of you may remember, I have been having a heck of a time keeping
trace elements in my show tank. As a result of these problems I have been
doing some tests on existing fertilizers to see how much it takes to get
a measurable level with my iron test kit. Here is the data that I have 
collected so far.

Fertilizers tested:
    Horizon Growers, 2 teaspons/55gal every other week
    Tetra FloraPride (iron-intensive), 10ml/10gal & 10ml/20gal @ water change
    Duplaplant-24, 1 drop/50l 
    Ferrovit, 2.5ml/38gal every 14 days

Test Kit: Red Sea iron test kit

All test were done with 1 liter of distilled water.

	Fertilizer	# Drops to get ~0.1ppm iron
	----------	---------------------------
	 Horizon		8 drops 
	 Tetra			12 drops 
	 Ferrovit		20 drops
	 Dupla			4 drops

* Note: The dropper I used took 120 drops to get 10ml.

	1. Seems to take A LOT more drops to get an iron reading then
	   I would have expected. 
		* I don't know how to read a test kit (could be)
		* My test kit is wrong (maybe)
		* These fertilizers really don't add much iron 

	2. It would really be nice to have someone run a similar test
	   on at least dupla with a different test kit to see if the
	   results are consistent.

	3. Dupla costs big $$$, but even by this crude test it is at least
	   twice as concentrated as the closest competitor.

	4. I dumped an entire 250ml bottle of ferrovit into my 180gal that
	   I have been having problems with and hardly got a trace of iron
	   to show up. I wouldn't waste the money. 

	5. To get a consistent iron reading on my 180 show tank that I have
	   been having problems with I used 2 bottles of Tetra (~$3.25ea) to 
	   get a readign of ~0.1ppm. 
	6. I didn't test the dupla tabs only the dupla drops alone. It could
	   very well be that the tabs add a lot more iron.

Since several people recommended testing the test kit with distilled water,
I decided to take up the challenge. Unfortunately, it is not at all clear 
what you should get for a reading. The recommended dosage doesn't even 
register (or was not possible with only 1 liter of water).  If anyone has
thoughts (especially hard data) on what we should expect in such a test
I would welcome the information.

Just thought that I would pass my test data along.