I use a gelatine mixture... the recipe on Knox Gelatine, Knox Blocks but substitute 2 cups sugar and 3 cups water for the 3 cups juice.
Make the Knox blocks in an 8x8 pan and chill overnight. Cut the pan into 1" cubes, add 1/4 pan to each 2 liter bottle. Let sit out to reach room temp (can hurry this but slows initital production). Add water to about 4" from the top, not hot water, just warm (a bit warmer if the blocks are straight from the fridge, still not hot you don't want to melt the cubes). Add 1/4 teaspoon yeast (best to activate the yeast by adding to a cup of water let it dissolve and add a pinch of sugar, then add this to the bottle and fill as before, Ok to skip this but it is faster to do this way)
I suggest using a bubble counter to scrub any stray yeast from the lines and to catch any foam ups.
I also suggest that you keep the generator and the attached bubble counter on top of the aquarium until the gas is bubbling into the tank. When the pressure is low, the water absorbs any CO2 and can actually be drawn up the gas line from the aquarium. If you have the bottle below the aquarium already, as soon as the water is over the rim of the tank, it races down to the generator and may increase pressure there so much as to break a seal and leak. Even if it holds, the generator cannot overcome the pressure from the column of water and so it stalls. Be sure the line is clear of all water before you put the generator under the aquarium.