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Substrate additives

I have a question about when and if to put fertilizer in a substrate.
Here are my situations:
	50 gal with Dupla Laterite and Kent's freshwater plant fertilizer
as a trace element mix and KNO3.  This tank is successful in that it has
very little algae, lots of plants about equal amounts of stem and Rhizome
plants. Initially this tank grew great and then after a month or so slowed
considerably.  I'm not exactly upset by this but I would like to be able
to speed up growth of the Vallisneria, Sagitteria and stem plants if 
conditions warrent.  There are 9 Pearl Gouramis, 3 ottos and 3 SAE's in
this tank with several Red Ramshorns and Malaysian Trumpet snails. There
is some long greem filamentous algae that I attribute to periodic over
fertilization with Iron in the Kent stuff and some periodic green spot
algae which I attribute to periodic over fertilizing with KNO3.  I must
confess I go by feel more than exact measurement and testing. :-0
	My hypothesis in this set up is that phosphate is in limited
supply.  Of the big three, I'm relatively sure that there is plenty of
Nitrogen and Potasium sp?  My Lamotte test detects no Phosphate and I did
not add any to the substrate.  I supplement with CO2 dropping the 4dkh and
7 dgh 7.5ph tap water to 6.5ish.  Now I may be deficient iron which would
hinder the plants ability to utilize Co2 but this sucker bubbles pretty
good so I assume that the plants are sucking it in and giving of the O2.
I am not all that familiar with Dupla's regiment but I think they add
tablets to the substrate from time to time, correct??  I followed
directions with the tablets that came with the Duplarit.
	Sorry for the long meandering post.  The other tank is set up
almost identically save the substrate is loam/worm poop/vermiculite and I
don't add any fertilizer.  Same plants much better growth.  Both have been
set up 6 months.

So let me be specic with my inquiries.  How important is phosphate in good
growth and given these scenarios would people agree that phosphate is my
limiting nutrient or something else??

Thank you in advance, any ideas would be appreciated.

Miles Morrissey   Easthampton, MA - USA
mmorriss at sophia_smith.edu