Here's why:

* Hydrogen peroxide is toxic to fish and other aquatic life. While it can break down chlorine, it also produces free radicals that can damage fish gills and other tissues.

* It can disrupt the beneficial bacteria in your filter. These bacteria are essential for breaking down waste and keeping your aquarium clean.

* It can oxidize other chemicals in your aquarium water. This can lead to unwanted side effects, such as clouding or discoloration.

Safe and effective methods to dechlorinate aquarium water:

* Dechlorinating solutions: These are readily available at pet stores and online. They contain chemicals that neutralize chlorine and chloramine, which is another common water treatment chemical.

* Activated carbon: While not as effective as dechlorinating solutions, activated carbon can absorb some chlorine and chloramine. It's often used in conjunction with a dechlorinating solution.

Important note: Before adding any new chemicals to your aquarium, always research and make sure they are safe for your specific fish and plants.