You can also use baking soda and vinegar to make CO2.

I have a spare 20 gal tank that I use. I put a small rubbermaid tub (12qt?) on one end with a gallon of vinegar in it. Place your rats, mice, ASF, etc.. in the tank. Then over the course of a few minutes slowly pour 10oz (volume) of baking soda in. If you do it to quickly it fizzes over and the rats freak out. Once you've poured all the baking soda in just let it sit for 10 or 15 minutes and when your all done you have nice humanely euthanized rats.

Also, CO2 is heavier than air so the container you use doesn't have to be air tight. I leave the 20gal open until all the baking soda is in then I put the mesh lid back on.

Last time I did it I only did 8 small rats but I could easily do 20, if not more in the tank.

The total cost for ingredients was $2.75

1 gallon of vinegar and 10oz of baking soda makes 20 gallons of CO2, so if your going to use a smaller or larger tank just do the conversion.