Well, we are all fish killers at some point in our hobby.
Poor fish.
If you add extra dechlor to the tank after rinsing, that is always added insurance.
Peroxide does not have something thast will do that.
Typically peroxide last about 1 hour in the tank before it decomposes.
Both bleach and H2O2 are strong oxidizers and will effects living tissues in a similar manner.
CO2 will kill fish too, many do that.
Adding too much baking soda too fast will also.
SeaChem Excel can also kill fish, at higher levels it's also a general biocide and is used to fix tissue samples.
Not too much else folks commonly add will do too much damage to fish.
Scaleless fish/catfish etc, tetras, Amano shrimps, these tend to be the "canary in the mine".
These are also the fish/critters I most like.
Just make sure you help someone else not make that mistake
Regards,
Tom Barr