The presure can be set very low. The variable is the size of the tank. Remember that co2 is heaver than air and will fill the tank forceing all of the o2 out the top. If you put a lid or cover on that tank to reduce air currents, the tank will stay full of co2 just like it would if you had filled it with water.

The correct way to do it would be to fill the tank with co2 first and then place the animal in the tank. That would help with that slowly running out of air feeling.

I've killed thousands of mice and rats this way but have not done rabbits. I think it should still be the same.
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Wade's Weptiles & Wodents

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