From The Aquarium Wiki
Excel is the shorted name for a very popular commercial product called Flourish Excel made by Seachem.
It is also popularly used to kill some species of Algae like Hair Algae when used regularly at 2-3x the recommended dosage. However this is not with the manufacturer's recommendation.
- If overdosed it may kill shrimp, fish, etc.
Polycycloglutaracetal - A form of Glutaraldehyde (C5H8O2) a potent disinfectant.
This product is widely available from the better local aquarium shops.
It is considerably more expensive outside of the USA due to export and shipping costs.
Some retailers selling Excel outside the USA.