Hydrogen peroxide

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About Hydrogen Peroxide

Typical cost £0.80 UK

Hydrogen peroxide (H2O2) is a very pale blue liquid which appears colourless in a dilute solution, slightly more viscous than water. It is a weak acid. It has very strong oxidizing properties and is therefore a powerful bleaching agent that has found uses as a disinfectant.

Even though it is a powerful and highly toxic chemical it can have a use in the aquarium if used in low dosages and with great care.

When added to water it rapidly breaks down into water and oxygen.

  • It is sensitive to light and rapidly breaks down in its presence.

The concentration this liquid is supplied by your chemist shop is usually around 2 to 9%. Though check with the instructions on your bottle. You'll need to know the concentration!

  • If you are using 3% hydrogen peroxide, each 1 ml added per 30 Litres (8 US gallons) will increase total peroxide levels by 1 mg/l. 15 mg/l per 48 hours is thought by many to be a fish safe concentration.

As an oxygen aid

You can purchase small white tablets which are a dilute solution of H2O2 and when the tablets dissolve they release oxygen to the water. This is intended to aid the transport of fish in bags of water.

Alternatively drip liquid H2O2 into the tank in very low concentrations. Typically 0.05 times your tank volume in litres assuming 3% solution. So a 60L tank would need 0.05 × 60L = 3ml per 12 hours.

Levels of H2O2 as an Oxygen aid per 12 hours
% solution × per litre × per US gallon example 60 Litres (15.9 US G.) tank example 250 Litres (66 US G.) tank
3% 0.05 0.187 3ml 12.5ml
6% 0.025 0.0935 1.5ml 6ml
9% 0.016 0.062 1ml 4ml

  • These cautious levels should be safe for nitrifying bacteria, fish, frogs and invertebrates.
  • In extreme emergencies a five times dose (0.25 instead of 0.05 per litre)[1] can be performed to save gasping fish. Though you may get some damage to your nitrifying bacteria or invertebrates.

As an algae killer

Concentrations of around 60ml of 3% H2O2 (30ml of 6%, 15ml of 9%) in a 250 Litres (66 US G.) tank directly applied (usually via a syringe) slowly over 5 minutes onto a clump of algae will kill it and then rapidly be diluted and converted into harmless oxygen and water. Observe the results and wait 48 hours before applying any more. Plants may be effected.

  • Over dosing will kill snails, shrimps, frogs, and fish!
  • Adding Barley Straw to ponds and aquariums has long been used to fight off algae. The straw decomposes and it is believed that it releases small quantities of Hydrogen peroxide which slowly kills the algae.[2]

As an antibacterial agent

Dilute concentrations added to the tank water can kill all bacteria in the water, including the nitrifying bacteria. It will also kill all micro-organisms, good and bad.

  • Taking an animal with a infected wound out of the water and dabbing the area with Hydrogen peroxide can be an effective way to remove bacteria from the area and it will remove dead decaying flesh from the wound.
But you have to balance this with stressing the animal by trying to catch it, removing it from the water and applying this chemical which may or may not cause the animal pain as it destroys bacteria and dead skin alike. A stressed animal can't heal itself. Plus how do you judge the amount of time to keep the animal out of water to ensure the chemical does it's work on the skin or fin, and determine the percentage of H2O2 to use?

As an oxidising agent on organic mulm

Dilute concentrations of H2O2 applied to a tank can breakdown buildup of mulm. See Oxygen aid.

As a safe alternative to using bleach

H2O2 in high levels (1 part H2O2 to 4 parts water) will perform the same job as bleach for cleaning equipment and empty aquariums. Unlike bleach it does not stick to surfaces and only breaks down into water and oxygen.

  • The density of 3% H2O2 is about 1,015g/L. Thus, 15ml is 15.22g, 3% of which is H2O2 (457 mg). Added to 30.3 Litres (8 US G.) yields 15.077 mg/L.

To achieve 15 mg/l you want to add 1500 mg of H2O2 which will require the addition of 49.28 ml of 3% H2O2.


  1. The Tropical Fishlopaedia book by Mary Bailey and Peter Burgess (on page 314) say 1-2ml of 3% solution per 10 Litres (2.6 US G.) (5-10ml per 37.9 Litres (10 US G.)) can be used as a oxygen aid. (ISBN 1-86054-102-X)
  2. Barley Straw - Algae Control Literature Analysis