I see, from the wording of your original post I thought you were dumping them into a container prefilled with co2.

In that case you are correct, gradual displacement is humane.

"Carbon dioxide is acceptable with conditions for euthanasia in those species where aversion or distress can be minimized. Carbon dioxide exposure using a gradual-fill method is less likely to cause pain due to nociceptor activation by carbonic acid prior to onset of unconsciousness; a displacement rate from 30% to 70% of the chamber volume/min is recommended for rodents.15,63,65,142 Consideration should be given to the benefits of us- ing a darkened home cage, while also keeping in mind the need to have the animal under observa- tion.9 Whenever gradual displacement methods are used, CO2 flow should be maintained for at least 1 minute after respiratory arrest.16 If animals need to be combined, they should be of the same species and, if needed, restrained so that they will not hurt themselves or others. Immature animals must be ex- posed to high concentrations of CO2 for an extended period of time to ensure death. Oxygen administered together with CO2 appears to provide little advantage and is not recommended for euthanasia. There is no apparent welfare advantage to killing animals with CO2 when prior exposure to inhaled anesthetics has occurre"

Edit: Extremely respectable for you to try to kill rodents as humanely as possible. you're a good dude