It is called Speciation. Meaning copper is often bounded up with organics, is much less toxic and has little effect at low levels. It is the free ionic copper that is so toxic. Kits often measure all forms of Copper.
Free Metal Ions
In seawater, copper is expected to be bound to organic materials. In one recent study of copper in natural seawater, more than 99.97% was bound to organic materials. Other metals, such as zinc, may not be as extensively chelated. In aquarium water, where the level of both metals and organics can be higher than in seawater, the percentage bound to organics may be even greater. Nevertheless, unchelated metals are very important. In the case of copper, for example, the unchelated copper ions may represent that portion of the total copper that is toxic to many organisms. These inorganic forms of copper and other metals are also expected to predominate in freshly mixed artificial salt water that has not been exposed to sources of organic materials.
Assuming that the organism does not take up the entire organic molecule to which the metal is attached (and many humic acids are known to be poorly taken up due to their refractory chemical nature), then chelated metals are often much less toxic than unchelated metals. In seawater, for example, the speciation of copper (i.e., whether and how it is chelated) is often much more important for understanding overall toxicity than is the total copper concentration.
The portions of metals in seawater that are not bound to organic materials are very complicated in their own right. Copper, for example, takes at least 7 different soluble inorganic forms in seawater. It is comprised of Cu++ (3.9% of the inorganic copper), CuOH+ (4.9%), Cu(OH)2 (2.2%), CuSO4 (1%), CuCO3 (73.8%), Cu(CO3)2-- (14.2%) and Cu(HCO3- )+ (0.1%). Similar complications hold for many of the metals that we are concerned with in this article.
Speciation and its Effect on Binding Metals in Aquaria
Since these metals take many different forms in aquaria, one must consider the nature of these different forms when developing methods for removing them. For example, metal ions such as Cu++ or Ni++ will never absorb at the air/water interface to permit selective removal by skimming. However, if the same metals were bound to an organic material that itself adsorbed to the air/water interface, the metals might well be exported by skimming. Similar concerns relate to claims about metal removal using activated carbon, polymeric ion exchange and complexation resins, and binding to inorganic materials like iron oxides and calcium carbonate. In fact, any proposed method of metal removal will be significantly impacted by the nature of the metal speciation. Depending on what is added to any particular aquarium, the speciation may actually vary from aquarium to aquarium, potentially making generalizations about them less useful.
