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Substrate Gold and fertilizer supplement

About ten days ago I set up a 40 gal. (water column) planted tank
using Substrate Gold as the substrate additive. I mixed it following
the recommended dose (two 24 oz. cans) with washed pool filter sand 
in the bottom 1 1/2" layer, added a sand-only 1 1/2" layer on top of
that, and topped with a 1/2" layer of 2-3 mm aquarium gravel. On the 2nd
day I added  the minimum recommended dose of FloraSan, and I repeated 
it the following day. I choose FloraSan because I don't want to bother
with PMDD for now, and this was the only product I was able to find that
completely discloses its formulation. So I know what I'm adding into
the tank. On the third day I started measuring Fe, and got a reading 
of around 0.2 ppm on my recently bought Seachem test. Whoa ! Isn't 
this too much ? It couldn't possibly come from the liquid fertilizer,
since the dose wasn't large by any standards. Tap water gives a 
barely detectable reading on the same test, and the test responds to
the reference solution with the correct color level. From the 3rd day
on I stopped adding anything to the tank, but I still keep reading between
0.15 and 0.2 ppm Fe. The obvious conclusion is that this iron must be
leaching from the substrate. 

So my questions are: when should one start using a liquid supplement
iron-based fertilizer in such a tank ? Should one assume that the
substrate will provide all the necessary trace elements besides Fe ?
Should I keep measuring Fe concentration and start to use the liquid
supplement and only when it noticeabily drops ? 

Ivo Busko
Baltimore, MD