Hydrogen Peroxide

[Val Shebeko]

I use a continuous water change system that takes out and puts back about 7000 mils of water over a 24 hour period. This in a 225 gal total water set up

I mix up a 45 gal batch of Reef Crystal solution at about 1.0255 concentration. This lasts for about 3.5 weeks or so. The water is in s closed ventilated Brute can and is constantly stirred.

Into the third week or so I get a “funky” slightly vinegar odor and sometimes a very slight layer of slime on the container walls.

On several occasions I’ve added a 32 oz can of hydrogen peroxide 3% to the full brute can and that seems to do the trick.

Question is in three parts.

1. Is this a dumb fix long term ?
2. How do I calculate how much peroxide can be added before I do harm in the tank system?
3. Is there some “better” additive that can be used in place of hydrogen peroxide?

I thank all of you very much.

Val

 

[Salty_Northerner]

From my understanding hydrogen peroxide breaks down pretty quickly and I don't think there's any issue but @Randy Holmes-Farley will set ya on the correct answer

 

[Val Shebeko]

From my understanding hydrogen peroxide breaks down pretty quickly and I don't think there's any issue but @Randy Holmes-Farley will set ya on the correct answer

That was my clever plan. To get the good Doctor to set me on the correct path.

 

[Salty_Northerner]

That was my clever plan. To get the good Doctor to set me on the correct path.

Lol here, I plugged your post into chat GPT and it spat this out.

Got it — you’re doing continuous water changes (~7 L/day) on a 225 g system with Reef Crystals stored in a 45 g Brute can for ~3.5 weeks. The odor + film you’re seeing is very typical of organics and bacteria slowly multiplying in stored saltwater (especially mixes like Reef Crystals that have vitamins/organics). Let’s walk through your three questions carefully.


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1. Is hydrogen peroxide a “dumb fix” long-term?

Not dumb — but it’s a band-aid rather than a root-level solution.

Peroxide oxidizes the organics and bacteria, which clears the smell/film, but it doesn’t prevent them from forming in the first place.

The fact that it works suggests your problem is bacterial growth fueled by trace organics in Reef Crystals.

Long-term, repeated peroxide additions shouldn’t cause catastrophic harm, but it’s not ideal because you can’t precisely control residual peroxide going into the tank.



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2. How to calculate safe peroxide dosing?

Let’s run the math with your example:

Brute can volume = 45 gallons ≈ 170 L.

You added 32 oz (946 mL) of 3% H₂O₂.


3% H₂O₂ = 30 g/L = 30,000 mg/L of H₂O₂.

946 mL × 30,000 mg/L = 28,380 mg of H₂O₂ total.

Spread into 170 L → ~167 mg/L H₂O₂ concentration in the Brute can.


Now, H₂O₂ decomposes relatively quickly in saltwater, especially in the presence of light, heat, and trace metals. By the time it reaches your tank via continuous water change, residual peroxide is usually negligible.

For reference:

Reef aquarists sometimes directly dose 1–5 mg/L H₂O₂ to the display for algae/dino control.

Your stored water is ~167 mg/L initially, but only a tiny fraction (~7 L/day out of 170 L) enters the system, and much of it will degrade before that. That’s why you haven’t seen livestock harm.


Still, it’s an order of magnitude higher than needed to sanitize water. You could likely use 1/10th to 1/20th that amount and still keep the storage container clean.


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3. Better alternatives to peroxide?

Yes — some reefers tackle this differently:

Shorter storage time – mix smaller batches so the water doesn’t sit 3+ weeks.

Stronger circulation/aeration – keeps water oxygenated, discouraging anaerobic films.

UV sterilizer in the mixing can – runs a few hours a day, keeps bacteria in check without additives.

RODI flush before mixing – make sure your source water isn’t seeding the container with bacteria.

Salt choice – Reef Crystals is notorious for “going funky” if stored too long. A lower-organics salt (like Instant Ocean, Tropic Marin Pro, or Aquaforest Reef Salt) stays cleaner in storage.

Sodium hypochlorite (bleach) – some aquaculture facilities use a trace amount, but it’s risky unless you can neutralize with sodium thiosulfate before using.


So peroxide is a practical fix, but UV + better salt mix is usually the “cleanest” long-term solution.


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Summary:

Not dumb, but a blunt tool.

You’re using far more H₂O₂ than necessary — 1/10th your current amount would likely suffice.

A better long-term fix is smaller batches, different salt mix, or UV sterilization in the can.



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Do you want me to calculate an exact “safe peroxide dose” you could use in your 45 g Brute so that, even if 10
0% carried through to the display, it would still be under known safe reef levels?

 

[Randy Holmes-Farley Verified Community Expert]

My concern with doing this has to do with trace elements reacting with the peroxide and changing form, perhaps into things that are less soluble (such as ferric vs ferrous iron).

That said, I do not know for sure it is a problem.

 

[Val Shebeko]

Yes, that was one of my next questions. What does bleach / peroxide / UV do to the underlying water chemistry.

I know people use inline UV in reef tanks to kill off unwelcome organisms in the water column. So perhaps the UV would cause the least amount of unwanted chemical interactions?

 

[Val Shebeko]

 

[Val Shebeko]

 

[Val Shebeko]

Something like this?

 

[Randy Holmes-Farley Verified Community Expert]

I don’t have a big concern about a UV in a mixing bin, but I think it is likely to get coated with gunk when used with a mix like RC.

I just use normal IO which does not support such growth.

 

[Val Shebeko]

IO?

 

[Val Shebeko]

IO?

 

[fishface NJ]

IO?

Instant Ocean (purple lid)

 

[Val Shebeko]

Doh 1 I knew

IO?

 

[Val Shebeko]

Thank you all for your help. Very much appreciated.

Val

 

[Val Shebeko]

Instant Ocean (purple lid)

This ?

 

[Val Shebeko]

Instant Ocean (purple lid)

This?

 

[fishface NJ]

This?

Yes that is what is referred to as IO

 

[Val Shebeko]

What I meant to ask but did not was, is this what us often referred to as “IO Purple”

 

[Fish Fan]

What I meant to ask but did not was, is this what us often referred to as “IO Purple”

Yes. What you linked is Instant Ocean Sea Salt, which comes in a purple box/pail, and is a very, very popular choice.

There is also Instant Ocean Reef Crystals, which comes in an orange box/pail, has some additional elements to help support coral growth, and is also a popular choice.

If your tank is newer and you don't yet have large colonies of stoney corals growing, regular Instant Ocean Sea Salt (purple box) is a fine choice, in my opinion. Just guessing, but I have to think Instant Ocean Sea Salt is the most popular salt mix among hobbyists, and many (all??) large public aquaria use IO Sea Salt (if they are not using natural sea water). I've tried other salt mixes, and now I'm back to IO Sea Salt personally.

I hope that helps!