They probably will not all die but they will suffer and begin to look bad. Some will eventually succomb to death. You will also probably get a massive algae outbreak. I learned that lesson when I let my DIY CO2 run out by accident. Couldn't figure out why the algae was coming back with a vengeance. Once I restarted the DIY cO2, the problem cleared up. You can grow plants without CO2. Just not the high light ones. You can also buy Metricide (glutaraldehyde) and use it daily to supplement. Then stop the CO2.