Sorry, this is an older thread but if I am reading this correctly than your 2L bottle is above the tank. I believe that this would allow the mix in the bottle to siphon into your tank, it goes from high to low position, thus when you do a water change you put the end of the syphon tube below and not above the tank. As mentioned earlier a check valve won't do anything for keeping the mix out of the tank, but will keep the tank water out of the mix. If you do have a check valve it's probably best to put the bottle below the tank and the check valve will keep the tank water out of the bottle and gravity will keep the co2 mix out of your aquarium.
what kind of silicone did you use, and did you let it completely dry and off-gas before you installed it?
Some types of silicone sealants off gas some nasty stuff (ammonia, probably formaldehyde, etc.) that you don't want in your aquarium.
No silicone sealant in my CO2 setup. I dabbled with it early on when trying to get a good seal for the airline running into the yeast bottle, but it didn't work so I ditched it in favour of a rubber gasket connector made for me by the guy in my local brewing/hydroponics store.