I followed some advice on a plant forum which went against the bottle of flourish excel. I read up a bit more of what it is... It is partly- https://en.wikipedia.org/wiki/Glutaraldehyde This is a sterilizer and fixative. Therefore, for sensitive fish, there is potentially organ failure... Therefore, while some consider it safe using 2-4x more than is suggested on the label in online forums, in many cases this is likely not safe. Please do not do this! Also, I wanted you to see this too, @TexasDomer and CindiL http://www.afscme.org/news/publications/workplace-health-and-safety/fact-sheets/glutaraldehyde http://www.cdc.gov/hicpac/Disinfection_Sterilization/7_0formaldehyde.html this one is more technical, but it has a lot of information on it as a bactericide. This ingredient is measured in the workplace to ensure a safe work environment... According to that website. .05ppm was the ceiling limit at one point. This means that was the maximum allowable level. That is extremely low, we are putting more than that in there. I think it is 2% by volume. I won't be using it anymore.