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Question regarding the safety of sodium thiosulfate to dechlorinate water

Hello everyone,

I haven't posted anything in months, as was waiting out winter here in Seattle and there was nothing really happening in my indoor system.  

Spring is upon us in the NW though, and I'm now back to paying more attention to my tank and hoping to plant crops soon, right now the tank has has been maturing with houseplants (which are doing AMAZING) and a grow light over the winter.

My concern that I can't find a definitive answer to is this - Is sodium thiosulfate dangerous to humans if used in system to de-chlorinate?  With rapid evaporation in my tank, and me not paying close attention since my business has been very busy, I've had to use de-chlorinator to top off my tank for the past few months.  I wasn't really thinking too much of it because such small amounts are used, but recently I started thinking I may have been flawed in my thinking.

There are a few posts about the chemical breakdown of chloramine using chemicals,  but not chlorine.  I've read one site (which now I can't find to post a ink to of course) saying that sodium thiosulfate helps the chlorine to degas quickly, and degasses itself, that seems too good to be true, lol.

My basic question that I need answered is will growing produce to consume in a tank that I've used de-chlorinator in have a chance of being harmful? 

I'm going to go set out a bucket of water to age while I wait for answers...


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I don't know about the sodium thiosulfate, but you can just fill a 5 gal bucket and leave it open and the clorine will just evaporate over the next 24 hours. Safest way to declorinate without chemicals or filters.


Gary, that works if your water is only treated with Chlorine, however many places around the country now treat with Chloramine which is more stable and therefore not as easy to "outgass" as the chlorine.  So, if you are on city water you should get a copy of the yearly water report to find out what your water utility uses for treatment.  It would take something more like three weeks of the water sitting out under UV light to effectively get rid of the Chloramine while you can simply bubble a bucket of water for 24 hours to get rid of most of the chlorine.


A carbon filter or activated carbon filter will work against either of those but the carbon only has a limited amount of chemicals it can take up before it needs to be replaced.

Good Info, Thanks.


Hey what about Reverse Osmosis? Man they're killin' us ain't they. Not to mention the Fluoride...

Gary Collins said:

Good Info, Thanks.


You need a pretty specialized filter to remove Flouride, I don't think RO will take care of that for you.  (I've heard of some filters that are supposedly able to remove flouride.)

Actually the Reverse Osmosis membranes themselves won't take care of chlorine either and most RO filters include pre-filters to take care of sediment and carbon filters to take care of chlorine and other chemicals and then the RO filter takes care of removing stuff like salt and calcium carbonate etc.

Last I checked it takes an electro something ion exchange method to remove the fluoride. fluoride molecules are smaller than H2O and so filtration is impossible. no wonder we're all sedated.


What about Seechem Prime?  Does anybody know if that is safe.  I've tried contacting the company with no luck yet (it is the weekend though).



To my knowledge, Chloram-X is the only aquarium water conditioner that says it is "non-toxic" to food fish and invertebrates---but check the MSDS. I personally would not use it with fish I am going to eat; it contains formaldehyde. And it binds ammonia; you don't want that.

You might want to check into Cloram-X. It's a bonding agent. It will require a testing kit that will test water treated with it. I found one on Amazon for no more than the traditional test kit prices. It takes very few grams to treat a whole lot of water (100's of gallons).

There is no known acute toxicity in humans or fish for the chemical compound sodium thiosulfate.

Here is some information from the manufacturer re: Cloram-X:

I've read through our threads, and I can't seem to find a good answer for whether thiosulfate (e.g. Seachem Prime) is harmful. The wiki page suggests not: thiosulfate is even used for certain medical purposes, as when a patient is fully of heavy metals and radioactive molecules in particular. The chemical can bind to heavy metals, which allows it to pass through the patient's kidneys -- useful.

I've been treating my water with the stuff for 2 years and neither the fish nor the plants seem to mind.

But I'm revisiting this now because I'm harvesting fish and they taste funny. Bad funny. Last year we harvested a few and they tasted really bad -- that was how I learned the hard way about purging my fish.

So I did it *right this time. And I pulled one fish (tilapia) last week ... we'd been feeding that tank corn meal only and not much of it for 10 days. The bad taste was not nearly as bad but still noticeable. And it got me to thinking.

Sodium thiosulfate is sulfur based. Could the sulfur be lingering in the fish? And could it be that plants don't generally take up sulfur such that their taste would change?

I would find it hard to believe that Seachem Prime would be harmful to humans because if it was harmful the fish would die or show signs of disease. But could it be that it affects flavor?

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