# Hydrogen peroxide disaster



## gcodrutv (24 May 2019)

Hi all,

I would like to share my recent experience using h2o2, aka hydrogen peroxide on my 300l tank.

On my current battle on the algae issue I have, I followed the advice that I received here on the forum and reading other people experiences on lowering the photoperiod and treat spot with h2o2 (15ml/dose) and Easy Carbo / Excel (12ml/dose) on alternate days combined with 2-3 x 50% water change per week.

Yesterday, I decided to increase the water change to 75% to expose to air more of the rocks and lower part of the affected plants.

So, I lowered the water (fish, shrimps and snails all in) and I did one of the biggest mistakes ever (if not the biggest). SPRAYING H2O2 ON LOW WATER CONTENT.

The intention was to spray peroxide on rock and wood to accelerate the process of killing algae. What I did not realize is most of the drops will end up in water. Basically, I was pouring 6% hydrogen peroxide all over my fish.

The result was *absolutely disastrous*, 20 out of 30 cardinal tetras died on spot and further 5 died in the course of next hours along with one of the 2 red phantom tetras.

What did not make any sense, the zebra danios are OK, the only molly same OK, red cherry shrimps and snails not showing any signs.

Lesson learned on the hard way, never ever dose h2o2 on lowered water. Always on full tank and after 50% water change.


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## Matty123 (24 May 2019)

gcodrutv said:


> Hi all,
> 
> I would like to share my recent experience using h2o2, aka hydrogen peroxide on my 300l tank.
> 
> ...



That’s uncanny! I did exactly the same and lost 30 fish (tetras) and nearly all my plants melted. This happened a month ago and since then I’ve changed everything from plants to substrate but yet to replace the deceased fish. Tank before disaster and tank now a week after set up.  







Sent from my iPhone using Tapatalk


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## gcodrutv (24 May 2019)

Your tank after disaster look stunning...
This morning, the plant were looking ok, no sign of melting. Will see tonight. Finger crossd..


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## Ueroe (24 May 2019)

zebra danios and molly are strong-endurance typical indeed. But red cherry shrimps are quite sensitive with changing water component. Hopefully red cherry shrimps and snails will ok forever  Finger crossed...


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## dw1305 (24 May 2019)

Hi all,





gcodrutv said:


> h2o2, aka hydrogen peroxide


I'm sorry for your loss. It is horrible when fish die.

The issue is that H2O2 is a really <"potent oxidising agent">. It is a very efficient biocide so it should be used with extreme caution and will burn fish gills, moss etc.

For me the risk factor means it doesn't have any place anywhere near a fish tank, but I'm <"pretty risk adverse">.

cheers Darrel


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## alto (24 May 2019)

gcodrutv said:


> treat spot with h2o2 (15ml/dose) and Easy Carbo / Excel (12ml/dose) on alternate days combined with 2-3 x 50% water change per week.



I’d suggest that alternate day doses with these two chemicals is also not the easiest thing for your livestock - both chemicals will be noticed by livestock, even at “safe” levels
Less challenging would be to use Excel during Week 1, then a week off (algae will be dying off during this time so daily water changes of 50% would be indicated), then Peroxide during Week 3

Note that when you spot dose with either Excel or Peroxide, should a fish or shrimp swim into that area, they will likely die (either then or later) ... I’ve even had an otocinclus pass after swimming directly into the Prime dose (watch the schlieren effect), since then I always pre-dilute any aquarium additives 

If you do choose to do this, perform at least a 50% water change before each dose - I’d actually do closer to 90% water change re various chemical byproducts (especially from both processes used alternately)
Of course, this also requires tank and tap water to be well matched to minimize stress from such large water changes 
(obviously drastically reduced water levels are stressful to fish, even with water matching)

(like Darrel I’ve never been able to imagine using peroxide in an aquarium - though I know it’s quite common in the Discus world  Every time I see a video demonstrating its use for algae control in planted aquaria, I wish for Warning Bullets on the same screen)


Sorry for your losses 

I suggest very conservative approach for the next couple weeks, 50% water changes are fine (as long as that’s the “norm” for your tank), but don’t add any new fertilizers, or (significantly) change fertilizer levels etc, maintain CO2 levels (if not for the algae issue I’d suggest a reduction in CO2 - depending on present levels) etc

Monitor cherry shrimp for reduced activity, opaqueness (not that you can do anything to treat aside from minimal stress and clean water)


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## gcodrutv (27 May 2019)

hi all,
thanks for your reply's. Just a quick update on the disaster.
None of the otto, zebra, cherry shrimp or assassin snails died since the chemical bomb.
Strangely, until today, none of the assassins were touching the shrimps. This evening I spot 6 assassins devouring 2 shrimps. That was shocking.   
Next day I did a 90% water change, just to be sure that all the chemicals are gone or almost gone. Fertilizer regime remain the same as co2 levels.
One thing I noticed is a very fast increase of water conductivity over 3 days. Most probably some of the plants are starting to melt. Every day I'm collecting dead leaves and small bits of plants.
The algae on the wood are starting to clear out same on the rocks, but some brush algae are flourishing and some are turning purple. Small patch of cyanobacteria is back close to the glass.
I will continue with small WC for the next few days, but I'll be away for one week.
Hoping no more fatalities.


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## dw1305 (28 May 2019)

Hi all





gcodrutv said:


> Hoping no more fatalities.


You should be OK now.





gcodrutv said:


> just to be sure that all the chemicals are gone or almost gone


You don't need to worry about a residue from the H2O2, it just decomposes to oxygen and water.






The problem is that a lot of oxygen is produced and that reacts with (burns out) any organic matter, including fish gills etc. 





gcodrutv said:


> Fertilizer regime remain the same as co2 levels.
> One thing I noticed is a very fast increase of water conductivity over 3 days. Most probably some of the plants are starting to melt.


Could be the plants melting, but it is likely to be that the lower plant mass isn't using as much of the fertiliser as well. I'd just up the water change volume a little bit.

cheers Darrel


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## Tim Harrison (28 May 2019)

gcodrutv said:


> Basically, I was pouring 6% hydrogen peroxide all over my fish


I think most folk use 2% H2O2 for controlling algae, 3% at the most.


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## Tom Michael (28 May 2019)

Agreed- I use 3 percent when spot dosing, never had any issues with fish or shrimp, although I am careful where I dose. I also turn my filters off for one hour. Personally I prefer than liquid carbon, which my moss hated and I have read is nasty stuff. If your spot dosing 3 percent h2o2 and that isn't doing the trick probably best to re look at the basics...co2 levels, flow, ferts etc.


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## Franks (28 May 2019)

I use H2o2 as a band aid when my tank goes out of balance until I work out what’s shifted. 

Just be careful using this, it’ll kill your good bacteria and livestock if too much is applied. Use with filters off and use prior to water change I.e a few days before your water change, not with a fresh water change. 

I spot treat using a 5ml needle syringe. It killed off Cyanobacteria in 24 hours. A good technique is to inject minute amounts just under the substrate layer. 

I dose 20ml of 6% in my tank which holds 155ltr when COMPLETELY empty on day 1 and then day 3 and the filter is off for around an hour while it all - I’ve never had to persist longer treatment courses. When my tank is stocked with substrate, hardscape, plants etc the water volume is reduced to 130 litres so bear this in mind. Filter volume is not calculated because it’s off during treatment.


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## tiger15 (29 May 2019)

Peroxide transforms quickly into H2O and O2 when exposed to air.  Any dripping into water is diluted into harmless concentration to fish and invert if not already been vaporized as O2.  The fact that your more sensitive inverts are not dead is a proof that you may be blaming  the wrong thing for the demise of your cardinal.

I can speak with confidence because I have been doing 75% WC on weekly basis and spray 3% Peroxide on all exposed plants, rock and hard surfaces.  Your use of 6% is double my strength  but I doubt that it makes any difference as your other fish and all inverts are unharmed.  Many people dose peroxide during power outage to replenish oxygen and there is a commercial slow release peroxide pills sold for fish shipment.  So I think it must be something else that  killed your cardinal.  Did you also dose excel during water change?


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## gcodrutv (29 May 2019)

@tiger15, I did not dose anything else during the water change. In the day I dose H2O2, I do not dose anything else, same with EasyCarbo. I do not mix them. 
Today I got some Excel deliverd. I was thinking to give it a try.

I was dosing 6% of H2O2 in the past, but never on low water.


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## tiger15 (29 May 2019)

Did you dose or spot treat with peroxide above water.  It's a big difference to dose or spot treat under water which can cause stagnation of high concentrations at low water level. 

I once dosed Excel under water at WC and  a large African cichlid, adjacent to my dosing point, lied dead for hours before recovery.  At low stagnant water level, it's easy to create high local concentrations specially with Excel which degrades much slower than peroxide.


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## Easternlethal (1 Jun 2019)

I too spray h202 every week when i do my 70% water change, all over the glass, in my empty sump, overflow etc. Its my detergent for getting rid of diatoms and algae and i sometimes spray if on my toothbrush and use it to brush all my pipes also. I have to say I have never experienced any fatalities. I tend to agree that if it's toxic then it would have affected all the fish and not just one type or another and I have cardinals, blacks, neons, shrimp, otos, corys and guppies.

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## stillc (19 Apr 2021)

Franks said:


> I use H2o2 as a band aid when my tank goes out of balance until I work out what’s shifted.
> 
> Just be careful using this, it’ll kill your good bacteria and livestock if too much is applied. Use with filters off and use prior to water change I.e a few days before your water change, not with a fresh water change.
> 
> ...


Are there any differences in what filter to use? Like I have a canister one, and it seems that the problem doesn't want to disappear.


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## milla (19 Apr 2021)

Wow just wow.   There's a skull & bones on the bottle of peroxide for a reason.  
If you regularly poisened your dog /cat the rspca would have you in trouble with the law.  Just saying good job fish aint cute and fluffy isn't it.


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## Andy Pierce (19 Apr 2021)

Bummer dude - that's a harsh lesson but thank you for sharing so others don't make the same mistake. 

Spot treat ≠ spray!  You can spot treat very effectively by either carefully dripping or swabbing your chosen poison (I use EasyCarbo) onto the problematic algae-covered surface.  The fuzziness of the algal growth means the liquid will easily and smoothly distribute over the surface covered with algae.  Not only does this keep the treatment from getting into the water while the water levels are reduced, it makes the treatment more concentrated right where you need it to be.  Make sure you don't use more total spot-treatment chemical than what you'd be happy with using systemically:  if you're using daily 12 ml Easy Carbo / Excel then that's the max amount you have to play with for the spot treatment.  Lower the water, do your spot treatment, add the replacement water.  Of course, do not ALSO then add your regular daily dose on top of the amount you added as spot treatment; the spot treatment is your daily dose, just applied to concentrated areas first.

This works really well from time-to-time (definitely no more than weekly) on isolated problematic 'spots' but if you have heavily contaminated broad surfaces, you have a systematic problem that is better addressed by systematic corrective methods:  more plants, reduced light, better flow, more CO2, more/better clean-up crew critters, etc.


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## dw1305 (19 Apr 2021)

Hi all, 


milla said:


> There's a skull & bones on the bottle of peroxide for a reason.


There is, but you can actually get the <"same effect">, even if you just have <"oxygen super-saturation">. Oxygen is, unsurprisingly, a strong oxidising agent.

cheers Darrel


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## dw1305 (19 Apr 2021)

Hi all, 
Thanks to @Andy Pierce, I now know why the hydrogen peroxide (H2O2) is more damaging because ........


Andy Pierce said:


> ......... peroxide reacts via free radical chemistry rather than straight up oxidation. Classical oxidation causes the oxidized material to lose an electron, but that doesn't happen with a reaction with peroxide. Instead the molecule that got attacked by the peroxide is now stuck with an extra highly reactive electron which it will try to hand off to another molecule which can lead to a long chain of reactions until the extra electron is finally scavenged by e.g. glutathione. The upshot is that peroxide is much more damaging chemically than oxygen is...........


and I'm definitely going to take his word for it.

cheers Darrel


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