Tonic recipe sounds nice, I may have to give that one a try.
Also I didn't know that trick with soda stream bottles, that's cool. But are you using liquid nitrogen to refill the bottles? IIRC liquid CO2 would need to be stored under a fair bit of pressure, otherwise it goes straight from ice to gas, but it's been a long time since I've had to know this stuff.
What's inside the sodastream bottles is liquid CO2. Yes, at atmospheric pressure it quickly boils off (which provides the 'gas' in your carbonated water).
as said, you can use either liquid CO2 or 'dry-ice' - in either case you can fill the canister by weight (technically mass) written on the side of the bottle. The 'skinny' Sodastream bottles take ~300g (again, use a scale) In both cases when you re-cap the bottle the pressure will build inside the bottle. Dry ice will convert to liquid CO2. This process absorbs heat, so the bottles will get quite cold until equilibrium inside the bottle is reached.
You can find food/medical grade CO2 in either forms all over the place. Praxair is a major distributor and has locations in most cities and large towns. Grocery stores frequently have dry-ice leftover from shipping, as do science labs and medical facilities (careful if you go this route that you know what they were packing). Home brewery stores and paint-ball stores carry liquid CO2 (though many paintball guns have gone to compressed air instead of CO2). Many will fill them for you though some will refuse due to liability or company policy.
Dry ice is simpler to work with at home - just remove the valve (releasing the pressure first!) and fill until the cylinder with crushed dry ice until it is at the weight written on the label, then replace the valve with a new o-ring. As with anytime you are wokring with tanks, check the hydro stamp on hte bottle neck and examine the bottle for any obvious damage before filling (e.g. cracks, major dents)
Do your own due diligence and don't just blindly follow the advice of some stranger on the internet. :-)
