AC will kill your plants
but only if you bury your plants under a couple of pounds of it.
seriously though, no. AC will not harm your tank. nor your plants. as darkblade pointed out it will remove approx 70 - 80% ( i forget the true percantage, but could find it if your interested ) Chealated Iron over a 96 hour period. so depending on your dosing methods, and if you are dosing Chelated iron in conjunction with other needed Macros a few times a week,
Then AC will...... clean / clarify your water, and remove odor.
and when left in for long periods, serve as a BioMedia in your filter.
Now as to why the plants are dying! What do they look like? old growth is yellow and dieing,
new growth is green and looks good? etc etc etc