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Sudden drop in dKH

Over the past two weeks I have been setting up my new 100 gallon plant tank.  
Yesterday I added the plants and eight large pieces of bog wood.  During the 
two weeks of set up as I ironed out mechanical problems with the CO2 and 
trickle filter, the dKH measured a consistent 3.0.  This morning the dKH 
measured 1.3.  This was using a Tetra KH test kit using 1 drop=1/3 dKH.  
Thinking the test kit might be in error, I reran the test using an Aquarium 
Pharmaceutical test kit which showed dKH greater than 1 and less than 2.

Question:  How could the dKH drop from 3 to 1.3 in 24 hours?

One hypothesis would be that the plants absorbed the CO3.  With CO2 injection 
that was providing 15ppm prior to the addition of the plants and bog wood I 
find it hard to believe the plants were starved for CO2.

Another hypothesis would be that I did not rinse off all of the potassium 
permanganate that I used to sterilize the plants and bog wood prior to 
putting them into the aquarium and that somehow the potassium permanganate 
combined with the CO3 and dropped it out of solution.  This is a wild guess 
since I have not idea what the chemical equation would be.

A third hypothesis is that the bog wood, purchased from Aquarium Driftwood, 
Semmes, Alabama, either absorbed the CO3 somehow or contained chemicals that 
combined with the CO3 and dropped it out of solution.

Any thoughts as to what might have happened?

My tank set up is as follows:

100 gallon (nominal) acrylic tank (94 gallons actual)
3 inches of flourite as substrate with Flourish tabs inserted
Wet dry trickle filter
Eheim CO2 diffuser located in wet dry filter 
Return from wet dry filter via two spray bars located under water so that 
tank surface water is not agitated
Lighting via 2 150watt HQI metal halide bulbs
Plants added yesterday
20 corkscrew val
20 dwarf Sag subulata
2 anubias nana
25 stems of mayaca
35 stems of didiplis diandra
30 stems of bacopa caroliniana
56 stems of egeria densa
35 stems of hygrophila difformis
40 stems of bacopa monnieri
1 Apon Ulvaceus

I use PMDD as fertilizer omitting the MgSO4 because my water is high in Mg 
(15 ppm) and using Potassium gluconate instead of K2SO4 since my water is 
also high in sulfates (85ppm).  Nitrate test this morning showed 11ppm of NO3 
(need to stop adding KNO3 until it drops to 5 ppm) with a pH of 6.7.  Using 
the CO2-pH-KH table CO2 is approximately 6ppm.

Best Regards,

Roger Gordon
San Diego, California