Jerry's Planted Aquarium Pages

Hydrogen Peroxide (H2O2) In the Aquarium


Hydrogen peroxide possesses some unusual and promising properties that might be utilized to great benefit in the aquarium hobby. Some of the principal benefits of H2O2 include:

This article will attempt to summarize some of the research into the usefulness of hydrogen peroxide in other applications, and to extrapolate from that information potentially useful knowledge in the realm of the aquarium hobby.


Any respectable theory must be capable of being proven false in some manner (i.e., the Falsification Principle). Another way to put it: no useful theory can ignore the potential criticisms that might be leveled against it. The same holds true for my theory that hydrogen peroxide can be used in the aquarium with myriad beneficial effects and no danger to the denizens of your aquatic habitat. In this section I will attempt to address the concerns you and others might have with dumping hydrogen peroxide into your aquarium.

The main concern I envisage people having is the potentially harmful effects that peroxide might have on the biological filter in an established aquarium. The bactericidal properties of peroxide are well known; that is why we dump it on open wounds after all. It is no stretch to imagine that adding it to the aquarium will have the same effect on the bacteria there that it has on the bacteria in the wounds we apply it to. To make that assumption, however, would be incorrect. When you apply peroxide to a wound you are applying it full strength (well, typically it's a 3% solution), but the concentrations we are talking about in aquarium applications are far from full strength. 3% hydrogen peroxide is 2,000 times stronger than the levels involved in this article. A study conducted to investigate the effects that hydrogen peroxide has on the development of biofilms on various substrates found that concentrations up to 16.5 mg/L did not inhibit the development of biofilms on the substrates tested (glass, concrete, and steel) (Momba 1998). If H2O2 concentrations of 16.5 mg/L do not inhibit biofilm development on the substrates indicated in the study, there is no reason to suppose that the same concentration will inhibit biofilm development on biological filter media in the aquarium.


If you are using standard 3% hydrogen peroxide, each 1 mL added per 8 gallons (1 mL per 30 L) will increase total peroxide levels by 1 mg/L.

Another misgiving regarding hydrogen peroxide in an aquarium might be potential harm to the aquatic life contained therein. The toxicity of H2O2 to fish and invertebrates has been studied in some detail. These studies seem to indicate that peroxide is potentially fatal to fish and aquatic invertebrates. All is not lost however; the concentrations required to produce fatalities are quite high and the duration of exposure quite long. This Material Safety Data Sheet (MSDS) on hydrogen peroxide contains the lowest concentration, that I was able to locate, and minimum exposure time required to produce 50% mortality in any aquatic organism. The organism is Daphnia pulex, and the concentration and duration were 2.4 mg/L and 48 hours respectively. Daphnia are known to be particularly sensitive to chemicals in the water; so much so that they are frequently used to directly measure pollution in waterways. The same MSDS also lists peroxide's effect on fathead minnows (Pimephales promelas) and found a 50% mortality after exposure to 16.4 mg/L for four straight days.

These may sound like worrisome effects, but empirical data suggests that peroxide degrades quickly in the aquatic environment of the aquarium. Hydrogen peroxide in the aquatic environment is subject to various reduction or oxidation processes and decomposes into water and oxygen. In water, hydrogen peroxide's half-life ranges from 8 hours to about 20 days (FMC Corp's MSDS on H2O2). Although I do not have any measurements available at this time I theorize that peroxide is rapidly consumed in the aquarium environment by fish waste, trace metals, dissolved organics, and other things. I have repeatedly added hydrogen peroxide to my tanks at the rate of approximately 15 mL/9 gallons. This yields an initial concentration of close to 15 mg/L. I have not seen any ill effects of this treatment to date. If we take the lower half-life as our guide, the peroxide concentration will have fallen below 0.25 mg/L within 48 hours. Notice that this is far below the concentration and exposure time for the fathead minnows.