3% Hydrogen Peroxide is 33,950 mg/L or 33,950 PPM Hydrogen Peroxide.

* * * Use of Hydrogen Peroxide in Finfish Aquaculture * * *

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Quote Originally Posted by http://www.koiphen.com/forums/showthread.php?109740-Hydrogen-peroxide-or-sodium-percarbonate-overdose-comments
The aquatic toxicity literature reports the 24 hour LC50 (Lethal Concentration when 50% of the fish die) of active pure hydrogen peroxide is 100 ppm. The 96 hour (4 day) LC50 is reported to be 22 ppm, the 2 hour LC 50 is reported to be typically 500 ppm.
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MSDS ECOLOGICAL INFORMATION

Chemical Fate and Pathway
No data are available.

Ecotoxicology

Data for HYDROGEN PEROXIDE 35% (ALLGRADES)

Aquatic toxicity data:
Slightly toxic. Fish 96 h LC50 between 10 - 37 mg/l

Aquatic invertebrates:
Moderately toxic. Daphnia magna (Water flea) EC50 = 7.7 mg/l
Moderately toxic. Daphnia pulex (Water flea) EC50 = 2.4 mg/l

Algae:
Highly toxic. EC50 = 0.85 mg/l

Microorganisms:
Slightly toxic. Bacteria EC50 = 30 mg/l