Elevated Iron and Manganese in my Fresh-mixed Sea Water... But How?

[dangles]

So I recently had a couple of unexpected (and rapid) fish deaths in quarantine. I ruled out the obvious initial causes (unhealthy fish, ammonia, temp/salinity/etc) and decided to send off an ICP test of my newly-mixed seawater to see if I could find a clue there.

The analysis came back *pretty much* clean except my iron and manganese were elevated (12 mcg/L and 51 mcg/L respectively) - both well above the expected levels. I don't think that explains the fish deaths, but that leaves me wondering... how did those levels get that high?

I'm on well water (a new system) and had a water analysis done prior to building the mixing station. Iron was undetectable, and manganese was 5 mcg/L. The water passes through a softener and then through my 7-stage RODI before filling the mixing bin where I add Tropic Marin Pro Reef salt.

I have not had any issues in my display thankfully, but how did those numbers get that high? Triton suggested the obvious culprits (overdosing trace - which I don't do, especially in my mixing bin), rusting metal components (there are none in my mixing bin), or some other contamination (which isn't helpful).

I'm leaning toward either a bad test (the ICP or the well water test I did), bad batch of salt (unlikely) or somehow my RODI system is malfunctioning.

Which stage would be responsible for filtering those elements out? Maybe one or more of my RODI stages needs replaced?

What else should I consider? And at what level is iron/manganese harmful?

Thanks!

 

[argonaut]

This is due to trickling agents. They break down quickly. You should therefore wait a few days with the icp and not send in freshly prepared water straight away.

 

[dangles]

This is due to trickling agents. They break down quickly. You should therefore wait a few days with the icp and not send in freshly prepared water straight away.

Trickling agents? Can you explain? My samples took approximately 5 days to arrive at the Triton testing facility. Does that make a difference?

 

[argonaut]

In addition to iron, manganese is also often used as an anti-caking agent in salt production. However, both elements are very sensitive to precipitation, i.e. after a few days there will no longer be much of them dissolved in your tank, especially if you have a high PO4 value.

Anti-caking agents (also known as anti-caking agents or anti-agglomeration agents) are separating agents that are added to crystalline substances such as sea salt to prevent the individual crystals from clumping together, primarily for the purpose of making them easier to use in machines. For example, sodium hexacyanidoferrate(II) (Na4[Fe(CN)6])

That is quite normal and every sea salt i know has elevated levels at the beginning.

 

[dangles]

In addition to iron, manganese is also often used as an anti-caking agent in salt production. However, both elements are very sensitive to precipitation, i.e. after a few days there will no longer be much of them dissolved in your tank, especially if you have a high PO4 value.

Anti-caking agents (also known as anti-caking agents or anti-agglomeration agents) are separating agents that are added to crystalline substances such as sea salt to prevent the individual crystals from clumping together, primarily for the purpose of making them easier to use in machines. For example, sodium hexacyanidoferrate(II) (Na4[Fe(CN)6])

That is quite normal and every sea salt i know has elevated levels at the beginning.

That makes a lot of sense! Thank you!

 

[Randy Holmes-Farley]

I would not assume those are anticaking agents, but regardless, I see no reason to be concerned. Some folks dose iron to levels far higher, and manganese is rapidly depleted.

 

[argonaut]

Here in Germany there is a well-known manufacturer of sea salt, Fauna Marin. They carry out an ICP analysis for every batch, e.g. here: https://www.faunamarin.de/wp-content/uploads/2024/01/Professional-Sea-Salt-23525.pdf
In case of manganese and iron they note: "Anti-caking agents have no bioactive effect and are removed by skimmers"

I am neither a manufacturer nor a chemist. I don't know whether these are really anti-caking agents or perhaps unavoidable impurities. However, it can be said that with any sea salt, regardless of the manufacturer, these rashes always occur at the beginning and that, as you have already said, it is not a problem.

 

[dangles]

Further confirmation for the “anti-caking agent” theory…

I had sent an email to TM and Hans-Werner Balling replied, also suggesting that as the likely explanation.

Thanks for all the input everybody.

 

[Randy Holmes-Farley]

I do not know if it is actually used in salt mixes, but the sodium hexacyanidoferrate(II) (Na4[Fe(CN)6]) will not be skimmed out.