co2

Im afraid that I am going to have to euthanize some rats for population control :(

So how much co2 will I need for 1 use? Im trying to figure out what size bottles (9 or 20oz) and how many.

If it matters they are all small 75-100 grams and maybe a few 200+ gram.

Just get a piece of dry ice from the grocery store, it's much cheaper.
Just spash a bit of water on the ice, and let the container fill up and the rats slow down, and once they're down, pour a lot of water over the ice, and they'll be passed out, by the time you do that, and they'll die.

Do you know how to construct a gas chamber for rats?

Thanks for the dry ice idea!
I have seen pics on how to construct a gas chamber.

Alrighty. Just make the chamber for the animal mostly air tight, you want the gas to build up, but still be able to escape. Also make it on the small side, I use a small plastic bin for mine, the smaller, the faster they will pass out, and the less room you have to fill with gas.

Just make sure to add the gas slowly at first, veeery slowly and watch the rats slow down, they'll eventually lay down, and breath heavily. They're basically unconscious or very close to it at this point, but their eyes will be open.

I know I'm repeating myself, but for good measure. . . Pour extra water on the ice once they've gone down, and fill the container up with 'smoke' from the ice. They'll be completely dead in a minute or so. Just make sure they all stop breathing before you stop putting in gas. You REALLY don't want them to be brain damaged and come back to life, it's painful and disorienting for them, and horrible to watch because they spaz. . . :(

Good luck!

Which is the quickest method and if there is pain, what is the least painful?

I really don't want to, but I have 6 small/md live rats and only 3 snakes that will eat that size.