Hydrogen peroxide is a fairly potent oxidizing agent. It's commonly sold at 3% potency (diluted in water) from drug stores, but can be found at higher concentrations in some places, up to 30%. It can be purchased in higher potencies for certain industrial or academic purposes.
It spontaneously dissociates in water (a process that can be enhanced by the use of certain catalytic metals) to create reactive radical oxygen species. These reactive molecules go on to attack the lipid membranes of any cellular life, including bacteria, algal, or multicellular organisms. This causes a chain reaction that can either compromise the integrity of the membrane (thereby killing the cell) or activate apoptotic pathways. Either way, dead cell. Multicellular organisms such as fish, plants, and inverts are usually less bothered by it as their outer epithelial cells can easily be replaced, and the animals usually have defensive mechanisms to prevent premature apoptosis. That being said, some research has suggested that long term use of hydrogen peroxide can hamper immune development.
When using any sort of chemical in a tank, it is important to consider the bystanders of such an act. While you might dump in peroxide to kill algae, it is an indiscriminate killer. It's commonly used in waste water treatment because it will pretty much kill everything. Your heterotrophic bacteria in the substrate and chemoautotrophic bacteria present in your filter could both potentially be at risk. While hydrogen peroxide is commonly used as a spot treatment, it is used in dosages roughly in line with Seachem excel, but the dosage you recommended is roughly fourty times the amount used in a spot application.
Both cloudy water and GW have manageable proximal causes. Even if you had some sort of a miracle cure, if you don't fix the main cause (or in the case of cloudy water, let the heterotrophic bacteria establish themselves), then your problem will eventually resurface. Let say you had two male cichlids in a tank, and one beats the snot out of the second, eventually putting him to within an inch of his life. So you remove him, put him in QT, nurse him back to health, and put him back in your tank. The next day you found he's been beat up again. Why? You didn't address the problem. Algae problems are the same way: people spend tons and tons of time dealing with the secondary and tertial algae issue, but refuse to address core issues.
So yea, I know what hydrogen peroxide is. In fact, I bet I know more about it than you. Your childish retorts are not appreciated. If you have issues with something, address the issues, not the poster.
