I doubt it was anything on the plants themselves that killed your fish but it could have been when planting them that you stirred up the substrate too much and caused an ammonia spike. I would check your parameters again and make sure you don't have any nitrites or ammonia.
All matter is merely energy condensed to a slow vibration, we are all one consciousness experimenting itself subjectively, there is no such thing as death life is only a dream which is an imagination of ourselves. Bill Hicks
ua hua's 90 gal.
ua hua's 5,000 gal. water garden