Chemist and koi keeper Dr. Roddy Conrad's views on the subject.
Baquacil Oxidizer is 27% strength hydrogen peroxide, useful for algae control when carefully dosed.
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.
I have actually dosed ponds full of algae with doses of hydrogen peroxide at 100 ppm actice concentrations many times and not seen any fish health problems as a result. However, when some of my friends have dosed new ponds or squeaky clean ponds with that dose, and there was nothing to consume the hydrogen peroxide, usually all the fish died in one to two days as the literature reports.
So what is a dose that will kill fish in a squeaky clean pond, you will surely ask.
Let's calculate that for an arbitrary 1000 gallons, then you can ratio that to your pond's volume.
1000 gallons times 8.3 pound per gallons gives 8300 pounds of water.
8300 pounds of water times .000022 to get 22 ppm (4 day toxicity limit) is then a pure hydrogen peroxide dose of 0.18 pounds. If the hydrogen peroxide comes as 27% strength as Baquacil Oxidizer, then 0.18 pounds divided by 0.27 = .66 pounds or .08 gallon or 1.2 cups. So to guarantee half the fish die in a squeaky clean pond a dose of 1.2 cups of 27% hydrogen peroxide per 1000 gallons could do it if the hydrogen peroxide last 4 days. If 3 quarts of 27% hydrogen peroxide is added to 4000 gallons, for example, to a squeaky clean pond, the dose is 48 ppm, half the 24 hour LC50 but twice the 4 day LC50, so some but not all of the fish are likely to die in a day or two. But if that same pond has a bunch of algae, then the hydrogen peroxide will all be consumed in a few hours and all will be well.
The case in point this week was indeed a charge of 3/4 of a gallon of Baquacil Oxidizer into a 4000 gallon pond where the algae was not bad enough to consume all the Baquacil, and many of the fish in the pond died in a few days.
If the fish are dying from a hydrogen peroxide overdose, a 90% water change is suggested to cut the hydrogen peroxide concentration a factor of 10. This presumes enough dechlor is on hand for the water exchange, ir not, an agent for reacting away the hydrogen peroxide is required.
The obvious best destruction agent for hydrogen peroxide is PP or potassium permanganate. If there is active hydrogen peroxide, the PP will be instantly consumed and will not turn the water purple or pink, so you will know when you are done destroying the hydrogen peroxide if you see purple or pink color that lasts a few minutes in the pond. Then add any dechlor to destroy any remaining PP when the pink or purple is seen. Then fish will stop dying.
