Dry Ice is pure CO². Soda water will not raise the level high enough to do anything but waste time and money. The hole idea of using enhanced gas (CO²) is to raise the level above 1500-ppm for extend periods of time and holding it there for many hours. If you are using CFL then don't waste your time with CO² they do not produce enough light to help the plant process the extra amount of CO² in the air.

You should have a sealed grow environment and have a closed exhaust to keep the gas in. You do not bring in fresh air from the outside when you use enhanced gas you just use fans to move it around your plants and fill the room.

You should change out the air once every 6 hours then fill the room back up with more CO² and start all over again.

Plus you do know the plant can only process the enhanced CO² during the light cycle?