Hello one800fish,
Yes hydrogen peroxide is not to be ingested by humans, but that doesn't mean it's toxic, just that it can cause problems for people if they drank it. Plants and humans are not the same thing either.
Although hydrogen peroxide will kill pathogens and all micro-organisms (depending on how much you use), that's not my reason for using it in my hydroponic systems. I would rather use ultraviolet (UV) light for that. I add hydrogen peroxide to the solution to add dissolved oxygen to the water (even when using an air
pump and stone). I add the hydrogen peroxide at one teaspoon (5mL) per gallon, that's a ratio of 758 to 1, not enough to harm the plants (except maybe seedlings), but still provide more oxygen for the roots. And yes it does help keep the growth of pathogens and micro-organisms (both good and bad) down.
I don't consider hydrogen peroxide to be much of an expense at all, one quart is enough for 189 gallons of
nutrient solution, and only cost me $1.06 at Wal-Mart (about $1.35 at the grocery store). Also I have yet to see a bottle of hydrogen peroxide that has any preservatives in it. If you read the label (both "Active Ingredients" and "Inactive Ingredients") there is nothing more than "hydrogen peroxide H2O2 (3%)" and "putrefied water" in the bottle.
P.S. As for me I don't believe copper kills pathogens, even if there was some truth to it, there would be a lot of variables in using it as to weather it was efficient at all. The water company certainly doesn't think copper kills pathogens. Houses are built with copper tubing inside the walls for both hot and cold lines, and it doesn't kill pathogens there. That's why the water company uses chemicals like chlorine to do that. If pathogens are a big concern, I would recommend using UV light. UV light has been used to purify water for about 100 years, and without adding any chemicals to the water either. Also easy to build/waterproof, and can be placed in the reservoir or in a inline overflow tube.