CO2 binds to hemoglobin molecules and it takes energy (ATP) and time to rid them of it. If they can't do it in time they'll die. Pretty much how people die of carbon monoxide poisoning, except in a much faster time since it has a stronger bond (triple instead of double I believe). With so many shrimp there will be a ton of CO2 in the water already.I'm not sure what you mean by this. You can inject shrimp tanks with CO2, even with dense populations of shrimp.