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Kelly wrote:  "I can't hold out any longer...
I've read numerous times here about folks adding K2SO4 to reduce
nitrates.  How does this work?  In all of my adventures at The Krib,
I've never run across this (not that it isn't there).  I don't
understand the relationship."
This is nothing other than our old theme of balancing nutrients.  Some
law or other (Liebniz?) says that plant growth will be limited by the
least available necessary nutrient.  We've probably had 100 comments on
the APD that potassium may be in short supply in our planted tanks.
I've made at least 25 of those comments over the last two years myself
(now 26 :-))!  If your tank is short on potassium (my tap water has 0.5
ppm) and has excess nitrates (and at least adequate other nutrients)
what will happen if you add 1/4th tsp. of K2SO4?  Increased plant growth
rates will result in reduced nitrates.  That is, the plants will have
the potassium they need (but formerly lacked) to increase their growth
rates which will also use up some of that excess nitrate, and result in
lower nitrate levels.
It gets a little bit trickier when you ask, having added the K2SO4, now
what nutrient is limited? You need sufficient trace elements and other
marcronutrients to drive the extra growth.  And if you don't have enough
light, you may just sit there with excess potassium and nitrate in your
tank.  So lighting, which provides the energy to drive photosynthesis,
is also a critical factor.  So as the growth rate increases (and
especially if it stalls out at some point which may indicate a
nutrient-limited condition) you may wish to experiment with a bit extra
of this or that.  One is working to achieve that elusive "balance" of
nutrients given one's initial water supply inputs, light levels, feeding
patterns, and fish load.  
The Dupla regime, and PMDD expressly, appear to strive for a
"phosphate-limited" condition.  This means that there are enough of all
the other nutrients needed for plant growth to drive the phosphate
levels to zero (through increased growth), at which point phosphate
becomes the limiting factor.  Having said that, I can tell you from
experience (previously reported on the APD) that tiny additions of
phosphate (I use potassium monobasic phosphate just for the hell of it
and because I'm such a broken record on potassium) in a high growth
rate, high light, high CO2 situation (which is Kelly's situation if
memory serves) have added quite a bit of sparkle to my tanks (how's that
for a scientific observation) without any noticeable adverse problems.

Steve Dixon in San Francisco