Overview of H2O2 First usenet metion: Jan 9, jan 11, 1991 Illistrated article showing syringe application 2 oz in 10 gal fishless, 2 oz in 35G kills shrimp and snails "Introduction to Fish Health Management" (US Fish and Wildlife Service, 2nd ed., 1995) recommends the use of hydrogen peroxide "at 250-500 ppm (100% active ingredient) for 15 minutes as an effective fungicide for incubating eggs and 250-500 ppm (30-60 minutes) for treating fish with external parasites" (page 108). - ibid interpret this as meaning one probably shouldn't expose fish to more than 250 ppm for time periods over one hour. Note that H2O2 is typically sold in pharmacies as (I think) a 2% solution or so. Assuming the density of a 2% solution of H2O2 is 1 g/ml, then one ml would contain 20 mg of H2O2, or 20,000 ppm. Using 4 oz per 35 gallons would result in about 140 ppm H2O2, which might be pushing the "upper end" for long-term exposure. I certainly wouldn't go any higher. I use 4 3% tbsp/gal of water and let is sit for 4 hrs before I use it with plants. 500-1000 ppm is the recommended concentration of 3% solution for hydroponics 1 ml/gal of 35% for hot tubs 1 oz per 10 gal Why barley straw works Effects of temperature on the production of hydrogen peroxide and volatile halocarbons by brackish-water algae. Relative density (water = 1): 1.4 (90%), 1.3 (70%) My observations: Straight 3% tried as a dip for 15secs. Kills red algae instantly, green thread algae seevrely set back, but will grow a week later. Oddly, when applied in a tank with a syringe it does kill green thread alage. Perhaps a longer dip is required? Crypt blassi was hurt as bad or worse than 1:19 bleach! Literature seems to suggest 250-500pm for animals, 500-1000 ppm for plants. Duration is unknown. Hornwort seemt to be the most sensitive plant, will try hornwort for one hour at 1000ppm. 10cc squirted into a 20 gal tank clears up all thread algae on java moss. Plant, amano shrimp and remashorn snail unaffected. 1:19 Bleach for 15 secs will not kill all red algae. It will return in a week or two.