Well I've never killed a plant using CO2 but I have killed them not using enough CO2. If it can be driven to levels toxic to plants you're more likely to kill your fauna way before that happens. Which enzymes are we talking about? Plants produce thousands of different kinds.
I dont know, it was on a topic that George posted on TFF, might be a bit out of date from when he wrote that
http://www.fishforums.net/content/Plant ... n-Dioxide/
Carbon Dioxide Levels and other Water Parameters
Recommended levels of CO2 are 20 to 35ppm. It is important that these levels remain fairly constant, as fluctuating levels are known to cause algae. Higher levels are sometimes used to fight off algae ? it is known that levels higher than 40ppm can block the enzyme production in the algae cells. However one must pay attention to the behaviour of the fish as levels higher than 35ppm are known to cause health issues.