I have set up a test DYI yeast-based CO2 system with the intention of using it in my 42 L planted tank & I have a few questions (new to the hobby so excuse my ignorance!). I used 1:1 sugar water solution with gelatin (about 400g in total) + another 300ml of water with about 2g of dried yeast mixed in, all in a 1 litre plastic bottle connected to a diffuser with a pvc tube, no valves. Lasted a week so far and there's still plenty of the sugar jelly for the yeast to go through.
Here are some action photos of the CO2 setup.


- What is the best way to make the bottle lid airtight? I drilled a hole in the lid, put the pvc tube through and then jammed lots of silicone glue on it from both sides after cleaning all the surfaces with alcohol. At first attempt it did not seal properly resulting in a leak and no pressure build up. Second attempt everything worked fine; I made the hole diameter smaller than the tubing and heated the tube to fit it through; currently CO2 is bubbling nicely through the diffuser, but the seal doesn't seem very durable -- the silicone glue seems to not bond well to the bottle lid. Is there a better way?
- I have read that some people recommend adding a second bottle to 'clean' the CO2 before feeding it to the tank. How important is this really? Is my thinking correct that this would be to dissolve the evaporated alcohol from the yeast fermentation in the middle bottle before entering the tank? If so I wonder if this is a big effect and whether it is necessary or more of a precaution. Reluctant to add the second bottle because it adds two more potential leak points.
- Should I worry about regulating the CO2 once I put the diffuser in the tank? I am planning to stock the tank with blue shrimp and neon tetra and I wouldn't want to gass any livestock. It is a 42 L tank so it is pretty small.. I was thinking I should maybe run the CO2 for a few weeks without any livestock and monitor the ph level and if that seems too low, then adjust the volume of the yeast solution accordingly. Does that sound like a good plan?
- Just hypothetically - what would happen if I didn't supply CO2 to the tank? The plants are mainly monte carlo and hairgrass but I am doing a dry start at the moment and I will have a full carpet before flooding (I have a journal thread for reference). Will lack of supplementing CO2 only stunt growth or can it cause the plants to die off completely? This is my first tank so I don't want to go high tech but I am worried the unregulated nature of the dyi setup might cause more harm than good to the future tank inhabitants..
Here are some action photos of the CO2 setup.

