30 quart is VERY big it will take a lot of CO2 to fill and take too long which means more panic. How many rats do you plan on euthanizing at the time? I recommend smaller kill tank 7 quarts it allows you to do 3 to 4 retire breeders at once and 10 to 30 smaller size rats.

Find a place that sell various gases for hospital or welding supply such as Airgas they will sell regulators and you will trade in your CO2 tank instead of refilling it which is better since those tanks must be re-certified every so often.

I have a 5 lbs tank and it cost me $25 to exchange.