Giancarlo Podio
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Eliminating Algae with H2O2

Before talking about Hydrogen Peroxide (H2O2) I'd like to mention that this is a method I use to clean out a tank after having corrected whatever problems were causing the algae in the first place. This article does not suggest using H2O2 as a long-term solution to algae problems, but a way to effectively clean your tank and equipment after having solved your algae problems.

H2O2 can be used in much the same way as many use bleach to clean their equipment out of the tank. The main advantage H2O2 has over bleach is the fact that you don't need to carefully wash any trace off before placing your equipment back in the tank, a quick rinse will be sufficient. But even more interesting, and the main focal point of this article, is using H2O2 in the tank itself to kill most kinds of algae. I performed the following experiments to test this out:

Spot Treatment
Tank: 55 gallon
Problem: Black Brush Algae (BBA)

First thing was to go to the drug store and purchase some H2O2:

The tank had undergone an increase in lighting and delays were encountered in getting the CO2 system running. As a result, BBA started to grow over driftwood, decorations and some of the slower growing plants such as Anubias. Once the CO2 was installed the BBA stopped spreading but what was there just didn't want to die! I tried a blackout and that didn't work so I decided to nuke it with H2O2.

First thing was obviously to prune all effected plants, I didn't want to treat the plants as pruning would have been more effective and would also promote the plant to grow back new leaves. Then I used an oral medicine syringe to help aim the H2O2 in the right direction, turned my filter off to reduce water movement and prolong the contact time between the effected areas and the H2O2. I then slowly squirted an patch of BBA until I had used 50ml of H2O2 (approx 1ml per gallon of water). I didn't want to use any more than this at this stage. All the plants started pearling like crazy due to the saturated levels of oxygen in the water. After 15-20 minutes I performed my regular weekly water change, around 25% of the water was changed. The filter was turned back on while the new water was added. The BBA bubbled like crazy for the rest of the night, within 24 hours it had turned a pinkish/red color and a couple days later my fish had completely eaten it all up. The following week I simply repeated the entire procedure on the next patch and so forth until all the BBA was eliminated. It took around 3 weeks to do it all and any equipment that could be pulled from the tank was soaked in a bucket with a stronger solution of H2O2, around 10ml per gallon of water.

I did not encounter any problems with my fish, plants or bacterial filter using this amount of H2O2 in the tank. I did not however have any floating plants in this tank so am not able to say if they would have been effected or not. It's also worth noting that this tank had a little thread algae among some of the plants that also disappeared during this period, perhaps due to the H2O2 in the water, not sure. Since then, I often use a couple of drops of H2O2 on my heater and spray bar during water changes to keep them clean but I haven't had any other problems in this tank to have to treat in the same way.

In another tank I used this exact same method to eliminate beard algae from foreground plants, worked like a charm!

Whole Tank Treatment
Tank: 10 gallon
Problem: Thread Algae

More recently I neglected one of my planted fry tanks, due to no water changes for months, no fertilizers and too much lighting this tank became very unhealthy plant wise and full of thread algae. My initial thought was to tear it down and start again, but first I wanted to see what effect a stronger solution of H2O2 than I had tried before would do. I therefore removed some of the thread algae with a tooth brush and left what was attached to the plants. Here is the tank as it looked prior to adding H2O2:

I left all inhabitants in the tank and did not turn the filter off this time. Inhabitants are snails, guppies, platties, a coridoras and many fry. Plants were in horrible shape to start with but include Hygrophilla Polisperma, Anubias, Java Ferns, Watersprite, Hornwart, Duckweed, Frogbit and Dwarf Sagg. I added 20ml of H2O2 (2ml per gallon) and waited around 30 minutes before doing a water change. I didn't notice much pearling at all this time, probably because the oxygen levels in this tank were not as high as the other tank to start with. I didn't notice any strange reactions from the fish or snails during treatment. I performed a 50% water change and left it at that.

The following day the algae looked a little softer and waving around a lot easier in the current generated from the filter, wasn't sure at this stage if this was successful or not. Another couple days passed and finally the thread algae just melted away, as did my Hornwart!

You can see the bare stem of Hornwart in the picture above, luckily it did start to grow back new shoots within a week. You can also see a couple of the last threads of algae waving and suffering in the current there. The other plants were not effected in any way. Some of the thicker patches of thread algae on the dwarf sagg were not effected as much as the ones present in the stronger current, but none the less, were certainly struggling. A second treatment would certainly be the final blow for them. In the weeks that followed I tested the water for ammonia and nitrite and was unable to detect any suggesting that the bacteria in the filter was not effected to a great extent. I don't feel safe saying that it wasn't effected at all because this is a well established tank which has otherwise been running healthy for over a year so there is a chance that the impact in a newer tank may be worse.

H2O2 is a great help in killing unwanted algae. It's not something you would want to use on a regular basis as I'm sure the plants will suffer but as a method of cleaning up a tank it allows you to do it without all the work involved in using bleach or side effects of most copper based algaecides and similar products. Plus it's dirt-cheap. Since, I have used H2O2 to clean out my hoses and just about any equipment that needs cleaning. Out of the tank, pure H2O2 is very effective. If you are considering using it to clean your fake plants, make sure you test it on a small section first as it may discolor.

Related links
- Introduction to Hydrogen Peroxide
- Marine Velvet - A Discussion of this Disease and its Available Treatment Options
- Fighting algae with Hydrogen Peroxide

I am aware of one incident where fish were lost in a cichlid tank using the above concentrations. It would seem that H2O2 is more dangerous at higher PH levels. Please always start off with a smaller dose before attempting any larger ones. Smaller, more frequent doses in my opinion are to be preferred over single larger doses. For spot treatments as indicated above there is no reason to try to reach a certain concentration in the tank itself, you are only increasing the risks by doing so.

Giancarlo Podio
Original article: 05/25/2004
Last updated: 03/17/2005