Hey, a good cheap way to increase co2 is by fermentation, take 1 half cup of yeast,easily bought dried, add that to a cup of suguar, to then 4 cups of luke water.HAS to be luke warm as to hot the yeast will die, to cold it wont start up, get it just right, luke tepid , goldy locks style.
In those rough preportions, it will bubble and produce co2, needs refill every few hours, twice a day i do it, so just empty half of it out and top back up with luke warm and a half as much sugar you put in the first time, top up water to orginal level.
it smells abity,well alot but works,,
Hope that helps,,take it easy..