The industrial use of Cidex, which includes the activator and the carbon compound gluteraldehyde mixed together creates a very powerful chemical used specifically to steralize medical instruments. At some point, someone figured out that without the chemical stimulator, Cidex is simply an industrial form of carbon and water. My understanding is that anything living is carbon based and complex aquatic plants thrive on specific doses of gluteraldehyde.
Apparently, primative plants, like algae can't tolerate it. So, gluteraldehyde, in high doses, is used to control algae growth. But, in doses similar to what's listed on the bottles of Excel Flourish, minus a little bit, because Cidex contains a higher percentage of "Glut", people who keep heavily planted tanks, notice pretty substantial plant growth and health. I use it sparingly, but my stem plants, that get most of their nutrients directly from the water grow fairly rapidly. Most of them, I have to cut back every couple of weeks or they overgrow the tank.
To answer your question about Cidex mixed with the activator, it would definitely kill your fish and you too if you ingested enough of it.