Excel is the shorted name for a very popular commercial product called Flourish Excel made by Seachem.
- If overdosed it may kill shrimp, fish, etc.
Polycycloglutaracetal - A form of Glutaraldehyde (C5H8O2) a potent disinfectant.
It's been noticed that even at recommended doses Excel kills of algae, and is popularly used to kill some species of Algae like Hair Algae at 2-3x the recommended dosage. However this is not with the manufacturer's recommendation.
Some Excel users have noticed a rise in nitrites or nitrates after introduction. Seachem's position is Excel shouldn't affect the nitrogen cycle of an aquarium, but there are online reports of this happening. (Possibly due to the product killing off algae).
Excel is reportedly hard on Invertebrates.
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.