Somethings sucking up my Fe

Hi everyone,

Two of my tanks have been sucking up Fe fertilizer at about double the
reccomended dose but not budging over 0.05 ppm (measured with the La Motte
low-range test kit).  One is a 55 gal tank with UGF and the other is a
25 gal tank without UGF.  In both tanks the plants look OK.  In the 55 gal
tank the cabomba is growing great & sending out shoots/roots.

I have two other tanks (both 10 gals, one with UGF, the other with laterite
in the substrate but no substrate circulation) in which I dose at about
1/10th the level that I'm adding to the other two to acheive 0.1-0.3 ppm Fe.

Does anybody know of good reasons why the first two tanks are sucking up

Maybe coincidentally, the 25 gal is going through something nasty right now:
I did a water change using water soaked with peat on Friday night & I thought
that maybe I rinsed out the two aquaclear 150 filters rather over-zealously
& got a slight bloom which I thought was maybe nitrites.  It cleared overnight
with extra aeration, but came back the next morning after I fed the fish.
It cleared a little the next night.  It did not really look like algae to
me since it was only a very light shade of brown & I thought maybe it was
diatoms until I saw a picture on the Krib of Erik Olsons tank with an algae
outbreak that looked similar: light brown fuzzy stuff all over the plants.
This morning the filters were gummed up with a slimy, dark-brown version of
the stuff & I rinsed them out & turned off the lights in the tank.  I also
shook some of the plants then took them out & rinsed them in the sink &
put them in other tanks - the slimy stuff was pretty easy to remove from
the plants but is now suspended in the water.  I will do a 50% water change
tonight & possilby rent a diatomaceous earth filter.  Nitrates are 3 ppm,
phosphates 0.4 ppm, pH 7.4, CO2 15 ppm, GH 6.   Any ideas as to what
caused this?  Is it really algae or diatoms (I could check some under the
microscope - what features distinguish the two?).