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In addition to my 75g tank, I've got a 29g that is my "low-tech" tank.

It's got 80w of light, large river-rock gravel (and 1/2 bag of Flourite
that I added later..), and DIY yeast CO2.  Well, last month, when the 
yeast mixture ran out, I decided to skip the CO2 injection.  I don't really
want the plants in that tank to grow crazy, because they quickly outgrow
the small tank.

Today after checking the pH of the 75g (6.8 @ 3dKH), I checked the
pH of the 29g.  Well, it was low.  About 6.6.  But I'm not adding
any CO2 to that tank.  Well, I just measured the KH, and it's
less than 1dKH.  I assume it's something like biogenic decalsification.

But reading on The Krib, I see a bunch of messages talking about rising
pH from biogenic decalsification.   So, which is it?  It would seem to me
that if the KH drops off, the pH should fall.

I haven't been good about doing frequent water changes on the 29g.  How
long should it take plants to strip out 3dKH?  I've got a spare 10g tank
set up with no fish, no CO2, and moderate light, and a bunch of clippings.
I filled this tank with 100% fresh tapwater just a week ago, and it's
KH is already less than 1.  

Is there something I should be adding to the tank to keep the KH up?
If I do more frequent water changes, I'm assuming the plants will keep
pulling carbonate out of the water, right?  

Does all this mean that a tank with live plants and lots of light 
NEEDS CO2 to prevent dropping KH?  I was always under the impression that
a planted tank would run just fine without CO2, just with slower growth.