Do not use this if you have live fish in the tank. Nothing should ever be added to the water except what is absolutely essential for the fish. "Safe" these days (at least in the thinking of hobby manufacturers and all too many hobbyists themselves) seems to mean the fish don't immediately expire...but that does not mean the substance is safe.
Hydrogen peroxide is a powerful oxidizer that has been used under field conditions to control I. multifiliis. High doses can cause gill damage leading to fish mortality (especially at high temperatures) (Schmidt et al. 2006; Noga, 2010; Picon-Camcho et al. 2011). I know, some will insist "high doses" means it is OK at low doses...not intelligent thinking when fish are involved.
There is only one safe way to deal with any problem algae, and that is to deal with the cause.
Hydrogen peroxide is a powerful oxidizer that has been used under field conditions to control I. multifiliis. High doses can cause gill damage leading to fish mortality (especially at high temperatures) (Schmidt et al. 2006; Noga, 2010; Picon-Camcho et al. 2011). I know, some will insist "high doses" means it is OK at low doses...not intelligent thinking when fish are involved.
There is only one safe way to deal with any problem algae, and that is to deal with the cause.