CO2 reactor

I wanted to ask your advice on my homemade CO2 reactor  I made weeks ago -
it seems to be hardly working and is in need of replacing (ie there is no
evidence of CO2 being bubbled into the reactor).   It is a standard 2 litre
softdrink bottle with a line into the tank into a small plastic softdrink
bottle, upended and weighted down  with several small holes about 1/3 of
the way up the bottle.

Last night I was suprised to notice the bottle was half filled with water
and half with CO2.  This morning the bottle was 3/4 filled with water and
1/4  with CO2.

What happened to the CO2?  Did it dissolve into the water? or does someone
have another explanation?