The rubber corks are airtight, but they will pop out if the fermentation gets out of control or there's a blockage. The jugs are made of heavy glass and the whole set up is designed for the purpose of fermentation.
The only difference between the DIY Co2 set up and the set up to make wine is the addition of fruit in the recipe. When you make wine, you put a little gadget called an airlock into the hole in the cork. It looks like a test tube with water in it. This allows Co2 to escape but keeps everything else out. The water in the fish tank would replace the airlock...
Do you think it would affect the fish negatively if I added fruit juice to the yeast and sugar? I like the idea of making small batches of specialty wines while adding CO2 to my tanks.