KH is used up slowly in a planted tank. Since pH falls as KH falls, pH can
crash when KH falls significantly below 1.0.

You can easily raise your KH to a comfortable 3 or 4 with baking soda if
you have an issue.

Start your reading he

http://www.csd.net/~cgadd/aqua/art_plant_co2chart.htm

http://www.qsl.net/w2wdx/aquaria/diyco2.html