> My SeaChem Fe kit appears to read the reference solution okay, but I'm not
> convinced that it's accurate in the low ranges we're interested in. 

It isn't.  Somone at Seachem needs to learn A=ecl.  

a= absorbance
e= molar extinction coef.
c= concentration
l= path length.

e is something that is a property of the method.
c is what you want to measure
l is what you increase when you want more sensitivity.

it is about that simple.

The "l" is a little short in the spot plates for low range test kits of
any kind. 

> When I
> add 5 ml of Tropica MasterGrow per each 50l of aquarium water, I get zero
> Fe readings from the SeaChem kit--which can't be right.  If I add enough
> chelated Fe to get a .1 - .2 ppm reading from the SeaChem kit, my tank
> breaks out into a rash of green spot algae (the plants are okay, but I have
> to work extra hard on the plexiglass the next Saturday).

OK, so here is a problem that is going to affect some iron methods.  In 
some cases, the chrmophore is not strong enough to pull the iron off the 
chelating agent.

You can do a potassium persulfate digestion to oxidize the chelating 
agent to oblivion, then test the digested matrix.  That is the optimal 
method for chelated metals.
> FWIW, I'm going to do the same exeriment with the Hach phosphate kit
> (PO-19) vs. the SeaChem phosphate kit.

I've done it, you can't read the seachem test below 0.1 mg/L.  This is a
generous assesment, but you may have better eyes.  (That is 0.1 mg/L as
PO4 ion, something like 0.03 mg/L P)

I don't know how everyone else feels, but Seachem test kits insult me.  I
can look at them and tell they aren't going to do what they say they will