Chemistry question (copper)

[kdrun76]

One of my fish tanks is set up to mimic Lake Tanglanika. I have a piece of Lime Stone in the tank in order to raise the KH, GH and pH.

pH is over 7.8 (the top my kit measures)
KH is 7 degrees (125 ppm)
GH is 6 degrees (107 ppm)

The tank has sea shells in the sand for the fish to play in (Shell dwellers). The seashells (made of calcium carbonate) get coated in bright green copper deposits. I can scratch the copper off and it will be back on in a day.

No other tanks have copper deposits on anything in the tank, and in this tank the deposits are only on the shells, none on the limestone (which is also calcium carbonate, but modified into calcite and aragonite.) I don't have copper stains in my drains or on any other surface of anything in the house.

Copper has an amazingly good reputation for being lethal to invertebrates, but this tank is full of snails (at least 4 species!)

So what's up with this? Why does this tank have copper precipitating out and the others don't, is the pH really that high? Why does it stick to the shells but not the lime stone? Is it precipitating out or is it just reacting with the seashells and coating them because the pH is right? And most importantly....should I care?

 

[LD50]

the only thing I can think of is that maybe you have Cu pipeing somwhere along your water supply that is releasing trace ammounts of a particular Cu ion that is precipitating when it comes into contact with the particular CaCO3 in those shells which may not be present in the lime stone. However this is all a wild guess without proper testing to figure out exactly what the green is and what is in the shells that is not in the rest of the tank. In the end if the life in the tank seems to be healthy I really would not worry about it.

 

[Lepisosteus platyrhincus]

^yep

mainly agreeing with his, if everything is fine dont worry

 

[RD.]

Copper has an amazingly good reputation for being lethal to invertebrates, but this tank is full of snails (at least 4 species!)

Karen, copper is only lethal to invertebrates at higher levels. Most natural bodies of water do in fact contain copper. The link below is an informative read on copper & aquatic life.
http://www.copper.org/publications/newsletters/innovations/1998/12/water_health.html

As others I could only speculate as to what's actually taking place within your tank, but personally I wouldn't be concerned.

Happy New Year!

Neil

 

[Westie]

If you really want to know, get a test kit. I know in my salt tanks that cu is lethal. I learned the hard way. Its beneficial in a fresh sytem yet, I'd think your snails would be first to go as mine were. The api test kit uses a high range, red sea and salifert have the best overall imho.

 

[RD.]

cu is only lethal in salt tanks, if a lethal level is present.

At lower levels, copper is actually beneficial in both fresh water & marine systems, even natural sea water contains copper.

 

[kdrun76]

I know copper precipitates out at high pH's and I know it bonds readily with calcareous substances. I find it odd that its bonded to the shells and not the limestone, but as has been suggested, it probably (not a hard guess either) has a rather different crystalline structure than the shells do. Apparently the shells have a better structure for bonding Copper than does the rock.

I think that tank has more snails than any other tank in the house, must be the levels are mighty low.