Some time ago I was talking to a friend of mine about copper and saltwater tanks. According to Albert Thiel, copper causes stress and accumulates in the fishes livers.
For a fish only tank, the best option to cure ICH (Cryptocaryon irritans)is hyposalinity, which I will discuss on another thread soon.

My friend wgscott has a PhD in Chemistry and teaches at University level.
Here it goes the Q&A about copper in a main tank (big NO NO):

-----------------------
Joaco:
My question is, what's the difference between citrated, chelated and ionic copper? how does ionic sublimation of copper works and is it true that copper will turn harmless and there won't be traces of it in a tank with calcareous stuff (substrate, death corals decoration)?
I always thought copper was a big NO for main tanks, and that it would leach back randomly, making it unsafe for corals and inverts later on.


Wgscott:
Copper can exist as a free ion in solution, as Cu2+. It can also exist as a complex with molecules that it has a high affinity for. These include citrate, and the oxygen atoms of the citrate interact directly with the copper, essentially surrounding it. This process chemists call chelation. So citrated copper is one form of chelated copper. There are some people who claim that chelated metal ions have a better chance of being absorbed by an organism than nonchelated ones. This may be true; I just don't know. At any rate, the other thing they do is to help keep the copper ion soluble.
Copper, in the absence of any other chelating molecule, will form complexes with water molecules, interacting with their oxygens rather than with those of (say) citrate. This is the normal way ions dissolve in water.
The structure can be somewhat stable and has a regular geometry (octahedral or tetragonal, for example, with the copper in the middle). Occasionally, the copper form a copper hydroxide, or even a copper oxide, under somewhat basic conditions (like those that exist in a marine aquarium, pH=8.3). These metal oxides and metal hydroxides become insoluble, and will precipitate out of solution onto the substrate, rocks, etc. This tends to happen slowly over the course of several days, and that is why you have to keep checking the copper concentration when you are using it to treat fish.
Since it is harder for this to happen if the copper is bound to citrate or some other chelating agent, these help the copper to stay in solution. However, it MIGHT be that the copper is less toxic (effective) in this form against parasites, so it is always a trade-off.
Anyway, the precipitation is what I think these people are refering to as sublimation (which is the wrong term, but copper doesn't sublime). The problem is that as the aquarium becomes more acidified (daily fluctuations, neglect, anoxic zones in the sand bed, etc), the copper can be reintroduced into solution in an uncontrolled manner, and then it can start killing everything in the aquarium.
So the short answer is yes, you NEVER want this stuff in your main system, because you can never get rid of it, and even if you think you have inactivated it by precipitation, this is a reversible process and it can always come back into solution in an unpredictable way.
Heavily chelated copper (whatever the hell that means) will still have the copper in an ionic state. It may be insoluble, but it hasn't been reduced to the neutral metallic form (like a penny), but even if it was, it still could be readily oxidized again.


Joaco:
‘If the copper test shows no traces, then the tank has no copper and it's safe for inverts and such.’. What do you think, can this tests fool you in any way? also, how about those resins by Kent that absorb copper? can you trust such a thing? I believe it would do a better job than carbon, but I guess the copper will still be leaching back randomly...


Wgscott:
The copper test relies on Cu++ binding more tightly to the complexing agent (don't remember what it is called) that turns color than it does to whatever the normal chelating substance (citrate or whatever). This will work provided the copper is not precipitated as copper oxide or hydroxide, which is insoluble. The test measures Cu++ in solution, but won't tell you if you have a completely undissolved penny in the tank or copper precipitated. As long as it remains precipitated, it isn't a problem, but remember that under more acidic conditions, it can go into solution.
Essentially the same arguments apply as for Calcium.
--------------------


Have a great day,

Joaco



------------------
Joaco's saltwater webpage Updated 27th April

but remember that under more acidic conditions, it can go into solution.

If your reef is acidic the copper will release, but a reef aquarium is kept on the alkalinity side, so would there still be a threat present?

IMO the danger is always present. You just need a part of the tank to go under more acidic conditions, and the copper will go back to solution there. Not wgscott said:

Occasionally, the copper form a copper hydroxide, or even a copper oxide, under somewhat basic conditions (like those that exist in a marine aquarium, pH=8.3)

If 8.3 is somewhat basic, I would risk to say e.g. 8.0 - 7.9 is already acidic, but I would have to double check with him.

But yes, the danger of copper going back into solution is always present, better be prepared for that, IMO running polyfilters or GAC to be there to remove the leaching copper (*if* and when that happens).

HTH

Joaco

------------------
Joaco's saltwater webpage Updated 10th May 2000