Increasing KH

(posted to rec.aquaria.freshwater.plants also)

 Well, 4 days away from setting up my 29 gal planted tank ...disaster
strikes !!! No, not really, but this is very frustrating.
 Here's the problem in detail:

-tap water tested regularly over the past 6 mo -->KH=4
-3 mos ago I set up a planted 10 gal with substrate from a local pet
store (dark gravel, mostly brown, grey, dark red and a few lighter
coloured grains, 1-3 mm size) At that time I did the vinegar test and
the gravel passed. For the past 3 mos on average I changed 3 gals
every 2 weeks and topped-off with tap water approx 4 cups/wk to
compensate for evaporation. Tested 3 times this week -->KH=6
I did not test the tank water all this time, just the tap.
-For the 29 gal I bought a 50 lb. bag of gravel, similar in
appearance, from a local pet superstore. The bag says "Red Flint
Filter Sands and Gravel" and "Drinking Water Treatment Materials
Classified by UL Labs". I did the vinegar test using 5% table vinegar
on various batch sizes and I got absolutely no fizz. I did the same
test on crushed coral and lots of fizz. 3 days ago I filled 3/4 of a
jar with this gravel and topped-off with tap water. Unfortunately I
didn't think of putting a tight cover on. Today, -->KH=5
 All the tests were done with a Tetra KH test using a 10 ml sample
size, resolution being .5 KH. I used the same bottle of reagent.

 So, is there any other explanation other than both gravels contain
lime ? Can evaporation count for the increasing hardness in my 10 gal
? The Dennerle book says that there is no danger of the aquarium water
becoming harder through condensation as long as at least twice the
amount lost by evaporation is changed every 2 wks. If my gravel
contains lime, will the hardness continue to increase in time or will
it stop at a certain value ? And, if lime is the only explanation,
does anybody know a good gravel source in the Metro Detroit area ?
 A timely answer will be much appreciated as the only time available
to setup this much-awaited  tank will be by the end of this week.    

 Didi Soichin
 didi at wwnet_com
 Westland, MI, USA