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K and TMG

My tap water averages 0.5 mg/l potassium.  I run high light, moderate
fish load, CO2 injected tanks.  A couple of years ago I started working
on the assumption that Tropica Master Grow should be supplemented with
additional potassium in my case.  I'm not certain my reasoning was/is
correct, but here it is:  I've never been clear how to interpret the
label of TMG, but it says that it is 0.79% K and 0.07% Fe.  It also says
that 100 ml of the stuff = 104 grams.  (The 4 grams must be the 'stuff'
added to the water. <g>)  I never quite figured out whether I should
multiply the percentages times 104 grams and then work out the dosing
amounts.  (I'm still interested in the answer to that question, if you
know. <g>) 

But I figured it was clear enough that the ratio of K to Fe is about 10
: 1.  As I was working out the dosing levels (I settled on 24 ml/week
based on observation (I use an Eheim Liquidoser to even out the dosing
because I am lazy) in a 125 gallon tank with a 40+ gallon weekly water
change) both Hach and LaMotte iron kits were able to pull out measurable
levels of iron, but always less than 0.1 mg/l.  I figured it was
unlikely that a manufacturer would target more than 0.1 mg/l of iron,
given the algae problems excess iron can cause in a planted aquarium.
So, it's been my guess that TMG probably targets 1.0 mg/l or less K.  I
doubt this is off by more than 100% given the havoc 0.2 mg/l iron has
caused me on occasion.  So I figure I probably have less than 1.0 mg/l
and almost certainly no more than 2.0 mg/l K using TMG alone.  Am I
right?

dr. dave (Heubert) has recommended a target for K of 5 - 10 mg/l in this
forum.  If I added about 1 teaspoon of KNO3 per week to this tank, I
would be adding about 5 mg/l K, but also about 10 mg/l nitrate (based on
110 gallons of water in the tank).  My tanks have never had measurable
nitrate without the addition of nitrate, but just in case this 2 : 1
ratio of nitrate to K is not quite ideal, I use K2SO4 for about ½ of the
macronutrient supplementation.  

In practice I'm not this scientific.  I add between ¼ and ½ tsp. of KNO3
and K2SO4 each week, depending on the look of the plants, level of green
spot algae on the glass, etc.  Neil asked about the symptoms leading one
to conclude additional K might help.  For me the plants just look
fresher and more vibrant with this macronutrient supplementation.  If I
go derelict for a few weeks nothing much happens.  The tanks may not
pearl as much and don't look quite as zesty and perky.  How's that for a
scientific, measurable observation! :-) 

Regards, Steve Dixon in cold, but sunny San Francisco

I know Karen is just being cautious and wants the newbies to succeed on
their first try, but it's been my experience that nitrate levels, even
as high as 25 mg/l, do not wreak havoc on an otherwise well-run tank.
Nitrate levels in Europe can be even higher than that.  I'm not
recommending anything like that, but I wonder how many folks out there
are futzing around with algae problems related to nitrate-limited
conditions without realizing it.  I did for at least two years!  I
wrongly thought I should drive nitrate readings to zero, if possible.
This is the central insight of the Sears/Conlin PMDD paper in my view.
I guess what I'm saying is that folks could start playing around with
small doses of KNO3 without needing to stay up nights with their bleach
solutions at the ready!  Just kidding, just kidding.