Problem Plant Tank

Hi, I continue to have problems with my plant tank.  I would appreciate any
help that I could get.  

Conditions before I had problems:  55 gallon tank.  One 40 watt Vitalite.
One large, beautiful, and healthy 5 year old Amazon Sword.  This sword plant
has been in this tank for 3 years without one problem under these conditions. 
GH= 3dh; KH= .5 to 1dh; Nitrate: 25 mg/l; pH= 6.0.  No CO2 injection
provided.  Fairly heavily stocked with Angelfish.  Filter media= Efimech,
efigrob, efisubstrat and efiflavin, Dick Boyd's Chemi-Pure, and filter
floss.  Algae= very light amount of green spot, and hair algae algae on the
glass only.  Fertilizers= Dupla laterite and Tetra Hilena Crypto tablets (
1-0-25 ).   

Present Problem Plant Tank Conditions:  same 55 gallon tank.  120 watts of
light provided by 1 Penn Plax Ultra Tri-Lux, 1 Coralife Trichromatic, and
one Vitalite.  CO2 provided by the yeast sugar method at one bubble per
second.  CO2 is bubbled into the intake my Eheim canister filter.  Algae=
very light amount of green spot and hair algae on the glass only. Plants=
the same old ( 1 ) Amazon sword, 1 wyteria, 3 hrgrophila polys, 2 very small
Java Ferns, 2 Onion Plants, 2 Apons.  These plants were all tried out (
experimental reasons mostly ) because I have made drastic chages in my tank
lighting level and CO2 conditions. With these changes in place I wanted to
see what would happen before I make a large plant purchase through a
mailorder company.  
Fish Presently In This Tank: a mixture of tetras, some dwarf cichlids, 4
clown loaches, ottos, and cory cats. 
Present readings: GH= 2dh; KH= 3.5dh; Nitrate= 12.5 mg/l; pH= 6.6; Tank
temp= 80 to 82 degrees F. Per advice from the list I have been adding baking
soda to bring my KH level up from the prior reading of .5 to 1dh.  The KH
has now been at 3.5dh. 
Per advice from the list I removed the Dick Boyd's Chemi-Pure because some
people thought that it was the reason for my GH level dropping from 3dh to
.5dh. I do realize it can and will remove nutrients from the water.
Chemi-Pure has been eliminated and even after making water chages, the GH
level is still not where it always used to be at.  It used to be at 3dh, but
is now at 2dh.      

Prior Conditions Before Following Advice From the List:  Lighting= the same;
CO2= the same; GH = a 3dh reading that eventually dropped to .5dh; KH= .5;
pH= 6.0 Nitrates= 25 mg/l.    

Fertilizers I have on hand and have used= Dupla laterite balls, Tetra Flora
Pride ( solubale potash 3%; and iron 0.02% ) and Tetra Hilena Cryto ( 1-0-25
) tablets, Aquarium Products Pond fertilizer with potash, chelated iron,
manganese sulfate, and zinc sulfate.  Analysis of Aquarium Products
Total Nitrogen= 0.00%, Available Phosphoric Acid= 0.00%, Soluble Potash= 1.00%
Aquarium Pharmaceuticals Aquatic Plant Stimulant with:  manganese, zinc, copper,
iron, molybdenum, and boron.  

Plant Conditions:  Hygro poly and wysteria= leaves pale becoming clear; leaf
veins stay mostly green before these "bad" leaves desinegrate and fall off.
My once old and healthy Amanzon Sword= gets yellow/brown spots on the inner
sections of the leaves only.  The tips and edges do not go bad.  Onion
Plant= just fine.  Java Fern= just fine.   

Recovery of Plants:  I took all of the hygro poly and wysteria that was
going down hill and placed them in the following condtions. These conditions
reversed the problems with these plants.  These once "bad" plants have made
a full recovery in the following conditions.  
Conditions: 30 gallon long tank; Water Temp: 80 degrees F.  One 20 watt
Coralife Trichromatic bulb; no CO2 injection provided; Fish= small number of
inbabitants. GH= 3dh; KH= .5 to 1dh; pH 6.0; and two very healthy amazon
swords already occupying this tank.  
Results: The leaf sections of the hygro poly and wysteria that were pale and
clear have almost completly "filled" back in with green color. They are
growing slow, but steadily.  I admitt that the very most bottom leaves of
the hygro poly and wysteria have turned yellow and gone bad, but the other
parts of these plants are very healthy now.  

I know this is alot to read, but I really need help with this.  I thought
the best way to get the help and advice I need was to provide all I could
tell you about the history of the tanks.  Thank you very much.  

"There are always exceptions to the rules"