My tip is use a check valve. How awful would it be if the mixture went into the water! Also, DO USE baking soda. My mix is slowing down in co2 production but it has been going 2 weeks. I tried without the baking soda and my mix lasted about 5 days before it totally burnt out. Also try to use silicone airline, as other airline materials may break down quicker... unless they're specifically for co2.
Be prepared. co2 does drop PH levels. Mine went from about 7 to 6.6. It does depend on the KH level of the water as to how much you can expect the PH to drop as KH is a PH buffer. (Think that's correct).
Also, make sure the 2 litre bottle (if you choose to use one that big) has a good fitting lid, I used a 500ml bottle lid on a 2 litre bottle as my original cap was not airtight.
The material I used to seal the area where I put the airline into the cap was bathroom silicone. It has sealed pretty nicely and still allows for a little flex. Oh and make sure the soda bottle is somewhere where it can't get knocked over easily, check valve will stop the fluid going through but still.
Also, I used a cigarette filter to diffuse into small bubbles and pushed this into the end of the airline. Works very well. Most people were talking about ripping actual cigarettes apart, but I just bought the actual filters from a shop.
Hopefully those tips help you. Plenty of guides into building DIY co2 on the internet.
