How to use hydrogen peroxide (H2O2) to kill algae

Written by: Marian Sterian
MasterLine Founder

H2O2 can make a noticeable difference if your aquarium is infested with algae like staghorn, fuzz, spirogyra, cyanobacteria and even BBA. You can use it as full aquarium treatment or as a spot treatment

How does H2O2 work

H2O2, or hydrogen peroxide, is a powerful oxidizing agent that can be used to kill algae in water. When added to water containing algae, H2O2 breaks down into water and oxygen, releasing free oxygen radicals in the process. These free oxygen radicals can then react with and destroy the cell walls and membranes of algae cells, causing them to die off.

There are many ways that you can use the H2O2 inside and outside the aquarium and it's important to start the treatment as fast as possible. If you allow the algae to grow too much, you will have a hard time getting rid of them. Certain types of algae can produce spores, which are specialized reproductive cells that can survive harsh environmental conditions and spread to new locations. The more algae an aquarium has the faster they will take over.

BBA on a snail's shell

Hydrogen peroxide can be easily found in every human pharmacy and it's not expensive. The concentration of the solution needs to be taken into consideration when you use it inside the aquarium. 3% is what i am suggesting as this is the most popular and easy to be dealt with. Higher concentrations can burn your skin and requires further calculations for a proper dose.

Different ways to use H2O2 inside and outside your aquarium

Please note that it's important to follow the exact steps in order to have a high chance of success.

When you are using the H2O2 inside the aquarium, double or triple the dose of micronutrients after the treatment, as the solution will also oxidize the chelators of Fe, Mn, Zn and Cu. With the chelators broken, these micronutrients will become unavailable for the plants and may cause washed out colors in the new growth of some species.

The hydrogen peroxide will last in the aquarium for 30-60 minutes, having the highest concentration immediately after adding it. As time passes by its concentration drops, as it reacts with organics.

Below i will explain the ways of using H2O2, that i have perfected over the years for all kinds of situations:

Inside the aquarium as a full treatment

This is most effective for algae like Staghorn. Every time i have used the H2O2 to kill Staghorn algae, i've had a 100% success rate.
It also makes a difference for algae like fuzz, spirogyra, brown hair algae and other hair algae types. For these algae, you might need to repeat the treatment after 3-4 days as they are not as sensitive to the solution as Staghorn is.

The steps are:
- while doing the maintenance, remove as much algae as possible manually (vacuum the algae using a hose, use a tooth brush, cut the affected leaves, replant the healthy tops of the plants, etc)
- make a 50% water change
- calculate the volume of the water left in the aquarium after removing 50%
- turn OFF the filters if they aren't already
- use 0.7 ml H2O2 for 1 liter of aquarium water. If the water left inside your aquarium is 100 liters, this means that you need to use 70 ml H2O2. Mix this quantity with 500-1000 ml of water
- while you stir the water in the aquarium with your left hand, gently pour the diluted H2O2 over the course of 20-30 seconds. Make sure you don't pour the entire solution in one spot. It's important for the solution to be spread equally in the entire aquarium
- if you have a currents pump (not filter), leave it running. It will help you to spread the H2O2
- allow 30 minutes for the H2O2 to do its work. After 30 minutes you can fill the aquarium to 100% and turn the filters ON. The reason why i am recommending you to turn OFF the filters is to increase the effectiveness of the hydrogen peroxide. With the filters running, the solution will lose efficiency by oxidizing the organic matter inside
- after 3-4 days Staghorn algae should be turning white or red and slowly start dying. Other green or brown hair algae should start decomposing as well. If the result is not satisfying enough, you can repeat the steps and this time use a stronger dose but not more than 1 ml H2O2 for 1 liter of aquarium water as it might also affect the animals inside the aquarium
- in order to have a higher chance for success, i am recommending that you decrease the light intensity and also the period it shines. Strong light gives energy to the algae
- you can also turn OFF the light for the following 2 or 3 days after the treatment. This should increase the success rate even more

Inside the aquarium as a spot treatment

This is effective for all types of algae. If you are using this type of treatment, make sure there are no plants or animals around. The solution will burn the plants and also affect any animals nearby. You can safely use the spot treatment when algae are growing on hardscape, technical equipment, substrate, glass or between the glass and substrate. Do not use on aquasoil substrate as it might break down the grains.

The steps are:
- fill a syringe with 3% H2O2 and connect the needle
- turn OFF your filters and currents in order for the solution to stay in contact with the algae as much as possible
- slowly release the contents of the syringe over the affected area
- refill the syringe and cover another area affected by algae
- make sure you do not overdose. Calculate the total volume of water inside your aquarium and multiply it by 0.7. For example, if you have 200 liters of water, make sure you don't use more than 140 ml 3% H2O2 in the same day. If needed, you can repeat in another day
- turn ON the currents and don't forget to double or triple the dose of micronutrients after at least one hour
- if you have cyanobacteria between the glass and the substrate, insert the needle in the affected area and release the H2O2. Make sure you cover the entire area. Kill the algae between the glass and the substrate and make sure you don't lift it in the water column or else it might start growing inside the aquarium. If you are using aquasoil, instead of H2O2, use the daily dose of MasterLine Carbo, or any other equivalent product. The hydrogen peroxide might break down the aquasoil grains as they also contain organics

Outside the aquarium

This method is also very effective against all types of algae. It involves removing the hardscape, technical equipment or any other static elements inside the aquarium and spraying hydrogen peroxide on them.

The steps are:
- remove technical equipment and if possible any wood and stones or other static elements, and clean them in the bathtub with a brush
- spray as much as possible hydrogen peroxide on them every 2-3 minutes for a total of 3-5 treatments. Make sure you cover all sides
- after 10 minutes, put the pieces back in the aquarium. Any algae that can still be seen, will slowly decompose inside the aquarium in the following days
- do a 50% water change after the algae have decomposed

Mastering all three ways of using hydrogen peroxide and keeping the aquarium as clean as possible, will help you keep it algae free.

It's important to note that while H2O2 can be effective in controlling algae growth, it can also be harmful to other aquatic organisms if not used properly. It's crucial to follow the recommended dosage and application instructions carefully to ensure that the treatment is having the desired effect without causing harm to other organisms.