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How much hydrogen peroxide is safe to add to a tank?


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How much hydrogen peroxide is safe to add to a tank to battle algae?

I added hydrogen peroxide to my tank today (in a localized spot) to try to get rid of staghorn algae. It looks like I added way too much. Most of my boras brigittae and a few of my pseudomugil gertrudae have died. 😭

I changed 1/2 the water, added more, and then changed 3/4 right away when I noticed the problem about 10 min after the H2O2 treatment. Hopefully I removed enough of it, but I expect to loose a few more fish. Shrimp seem to be fine. One nerite is upside down, so I am taking it out for now.

29 gallon tank

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Edited by AnsleyBleu
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On 11/19/2021 at 10:08 PM, AnsleyBleu said:

How much hydrogen peroxide is safe to add to a tank to battle algae?

I added hydrogen peroxide to my tank today (in a localized spot) to try to get rid of staghorn algae. It looks like I added way too much. Most of my boras brigittae and a few of my pseudomugil gertrudae have died. 😭

I changed 1/2 the water, added more, and then changed 3/4 right away when I noticed the problem about 10 min after the H2O2 treatment. Hopefully I removed enough of it, but I expect to loose a few more fish. Shrimp seem to be fine. One nerite is upside down, so I am taking it out for now.

29 gallon tank

IMG_2605.jpg

Sorry to hear about your struggles. If you're treating algae on hardscape, it is always advisable to remove the hardscape, treat with peroxide, wait a few minutes, wash off, then return to tank. It will die in a few days. Do not just add loads to H2O2 to a tank. A little bit goes a long way.

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I've never done it, but back in the '80's, a Guppy club in FL. issued one of their monthly bulletins that said to add one teaspoon of H2O2 per gallon of water to increase the amount of O2 in the water. If I'm treating for hair alga, I have H2O2 in a spray bottle that I can adjust the nozzle from a wide angle spray to a direct stream. I'll lift the affected item out of the tank to spray the alga and then I'll quickly put the object back into the tank. Then I'll watch the alga start giving off bubbles and turn red which means it's on it's way to being dead.

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I’ve used 3 mls per gallon pretty commonly and I’ve read up to 5 mls per gallon is fine.  5 mls per gallon is the same as @Gatormentioned.  So my 20 long that was giving me fits with excess algae can get at least 60 mls (2 ounces) and up to 100 mls, that’s over 3 ounces.  Peroxide breaks down pretty rapidly into oxygen (O2), and hydrogen (H2).  Too much at once can harm fish or shrimp if poured directly over them, but not usually just put into the tank unless you use a crazy amount.

It should be the 3% solution available in any drug store in the USA.  I stop the HOB’s, usually leave the air driven sponges on because it’s more hassle to stop those.  I squirt the solution directly on the areas I want to hit, sometimes repeat a couple times if that’s the only problem area in the tank (using my predetermined total, just concentrate it in one area).  I wait 10 minutes, then turn the filters/pumps back on.

I’ve done this several days in a row until the algae turns reddish and washed out looking.  Then I just wait for my clean up crew to work removing the dead algae.

I have definitely overdone peroxide dips for plants (outside the tank - both too strong and too long), but I don’t think you could with wood, especially if you rinsed afterwards.

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I’m so sorry for your loss. I know you must be devastated. 😢 I usually remove things to treat with peroxide so I cannot speak to adding it to the tank. I can speak on snails and h2o2 is a fast and effective snail and snail egg sac eradication method sprayed directly on plants for 15 seconds. Perhaps remove them before treatment to keep them safe. I found this out on accident removing algae. 

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On 11/20/2021 at 9:58 AM, AnsleyBleu said:

Thanks everyone. I had been adding liquid iron when I probably did not need to. That is when the staghorn showed up. I will be critically aware of quantities if I ever use this method in the future.

The liquid iron in your tank, and then the addition of H2O2, may have generated a chemical reaction where even a small dose of H2O2 may have been too much for sensitive species. 

I am so sorry for your loss, I don't think I have ever seen anyone ever post about the chemical reaction between iron and H2O2, and to be honest, I would not have thought to ask if you were using a high iron fert before sharing how to use an infant medicine syringe to focus the peroxide on specific areas of plants.

That Fe=>H2O2 reaction is what makes peroxide bubble when we pour it on a wound: the peroxide oxidizes the iron in our blood (the heme) and the oxidation process generally lifts any debris up and out of the wound... however, that kind of oxidative reaction in a fish's gills isn't so great. 

Thank you for sharing your experience, I will be more mindful when I recommend using a spot treatment of H2O2 to treat some algae growth.

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I meant to say that I had the staghorn algae appear after I had been adding liquid iron plus semi-weekly Easy Green. I had not added H2O2 and iron on the same day, but there was probably still some liquid iron in the water column. There was massive bubbling when I added the H2O2, but I probably added over half a larger sized bottle (kicking myself).

I believe I crashed my whole cycle. The water is cloudy today so I ran to the store and bought a bottle of starter nitrifying bacteria. Hopefully this will help and is not just "snake oil." I only have 4 fish left and shrimp are dying now. Ugh.

On the bright side, (not that there really is one) all the staghorn algae is definitely dead.

Edited by AnsleyBleu
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On 11/21/2021 at 6:58 PM, AnsleyBleu said:

I meant to say that I had the staghorn algae appear after I had been adding liquid iron plus semi-weekly Easy Green. I had not added H2O2 and iron on the same day, but there was probably still some liquid iron in the water column. There was massive bubbling when I added the H2O2, but I probably added over half a larger sized bottle (kicking myself).

I believe I crashed my whole cycle. The water is cloudy today so I ran to the store and bought a bottle of starter nitrifying bacteria. Hopefully this will help and is not just "snake oil." I only have 4 fish left and shrimp are dying now. Ugh.

On the bright side, (not that there really is one) all the staghorn algae is definitely dead.

You can't know what you don't know, and trust me, ALL OF US have a minimum of 1 "kicking myself" experiences... and most of us have more than one. 

Yes, the bubbling you saw when you were pouring was the iron / H2O2 interaction. For future reference, I have a bunch of infant medicine syringes... because I used to have a bunch of infants.

I have one dedicated to each tank specifically for testing my water (easier to fill the test tubes, and no cross contamination risks).

I have a syringe dedicated to each medication, and one dedicated for each water treatment (makes my life easier).

When treating algae, I turn off all filtration and aeration. I want the water to be still for at least 10 minutes, so I can use the least amount of H2O2 and still get maximum benefits. 

I'll fill the syringe with 5 cc of H2O2, and squirt a cc at a time onto each clump, and up to 5 cc / cubic inch of string algae or blackbeard algae (they seem to be the most stubborn).

Wait 10 minutes, and turn everything back on.

My snails take care of it after that.

In my pico tanks, I never use more than 5 cc.

In my 4' tanks, I only treat the tank if I plan on showing it. Otherwise the larger volume seems to prevent major algae outbreaks. 

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