Yesterday afternoon I made myself a CO
2 generator from a fizzy drinks bottle and some left-over air line bits'n'pieces.
Dry stuff: Drill a 5mm hole in the bottle top. Slice the end of the air-line into a point and, using pliers, drag the air-line a few cm through the bottle top. This gives a good enough seal for the meagre amount of pressure generated. Shove the other end of the air-line on to an unused air stone.
Wet stuff: Dissolve 1½ cups of sugar in a jug of water. Add the water to the bottle and top up to about ½ full (or ½ empty if you're feeling pessimistic). Shake well. Add 1½ tsp of fast-acting dry bread yeast and shake well.
Stand the bottle somewhere suitable, put the air-stone somewhere suitable..... and wait.
Remember; the CO
2 has to displace all the air in the bottle before it gets in to the water, so it will take a while.
IMPORTANT BIT: There is absolutely no control over the amount of CO
2 generated this way and it is entirely possible that it may cause a pH crash in your tank. You do need to be well up on the relationship between pH and KH and how this affects the CO
2 content of your water, and have test kits for both pH and KH. Dissolved CO
2 chart
here.
I'd only got a short bit of air-line so this isn't the final resting place if I decide to keep it. However; as this is the hardware section of the forum I'll discuss the efficaciousness (or otherwise) of the CO
2 addition elsewhere.
If the bottle is below the tank then an in-line non-return valve would be a good idea.
DISCLAIMER: I am no way responsible for the sticky mess that may ensue from any attempt to recreate the above contraption.
And, before you ask.......