I found this on the web
http://www.barrreport.com/general-plant ... e-nh4.html
The active ingredient in excel is polycycloglutaracetal, which is patented by Seachem. Polycycloglutaracetal is derived from glutaraldehyde, which is toxic and commonly used to sterilize medical equipment. According to Seachem, polycycloglutaracetal's molecular makeup contains a 5 carbon chain (glutaraldehyde is C5H8O2) which is accessible to plants.