The major problem with DIY CO2 isn't having too little CO2, but having a fluctuating concentration of CO2 in the water, which signals algae to start growing. With low light, which you probably have, algae grow slowly, giving you plenty of time to get rid of it before it becomes a problem.
If you stop the CO2, all plants will grow slower, some will grow much slower. Crypts may melt entirely. Some plants that are borderline on light, now growing because CO2 enables them to grow with less light, will just die off.