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Newbie dosing and CO2 question

Hello all,

I'm about to jump completely in and do my first heavily planted
tank. I thought I would round up the fertilizers I would need
ahead of time.  A local hydroponics store has monopotassium phosphate
which I assume is the same as potassium monobasic phosphate
(KH2PO4), and potassium nitrate KNO3. However, they can't supply
a reasonable quantity of potassium sulphate (I would have to buy
25 kg !!!  at $4.75 CDN/kg !!). This store does have magnesium
sulphate in reasonable quantities, however.

According to the city waterworks lab person I spoke with, our
water has pH 7.5 - 8.2 
          total hardness as CaCO3    140-175 mg/L (=ppm)
          total alkalinity as CaCO3  106-127 mg/L
          carbonate                        0 mg/L
          bicarbonate                106-127 mg/L
          calcium                      36-48 mg/L
          magnesium                    12-13 mg/L
          sulphate                     35-44 mg/L

In this kind of water, do I need to dose for sulphate, and if so, can
I safely use magnesium phosphate instead of potassium sulphate? If it 
matters, the lighting will be 2-3 W/g (a small 10 g tank).

Regarding adding CO2, the procedure as I understand it is: 
  - determine the alkalinity and the pH (despite the info above,
    I've obtained test kits to do this more accurately)  
  - look on the KH-pH-CO2 chart to determine what pH corresponds
    to ~25 mg/L CO2
  - add CO2 until pH drops to this level

Is this correct?

Also, I have a 1 year old, 27 gallon, 24" tall tank that has about
20 small to medium fish in it. It has an Eclipse II canopy that
provides a biowheel filter and about 30W of fluorescent light.
After having read the APD for a little while, I added some java
fern, 'christmas' moss, and some water sprite - much to the delight 
of my fish. I am thinking of adding anubias and crypts. 

Will adding CO2 benefit this tank with this low level of light?
And while I am at it, I want to be doubly sure: will the pH swing
harm my treasured fish?

Thank you very much for your help! Like others who have mentioned
it, I am grateful for all the experienced aquagardeners on
this list providing their insights and answers to newbie
questions like the ones above.

T. =8)
Titus Mathews Jr.