Trace elements in the Aquarium

First of all, it is necessary to distinguish the importance of the different elements present in larger quantities (N, P, S, K, Ca, Mg: 0.05 - 8 % of solids) and trace elements (Fe, Mn, Zn, Cu, Mo, Co, Ni, B: 0.2 - 7000 ppm of solids). There are several starting points for such definitions, but all of them have the same problem; it is difficult to make a clear distinction

Structural elements ↔ Functional elements

It is difficult to make a consistent distinction, as some substances have to be assigned to both groups. Magnesium, for example, is both contained in organic structures and effective free in the cytosol.

Metals ↔ Non-metals

Macro-elements ↔ Micro-elements

All these divisions, however, cannot be consistently upheld and there are overlaps.

Here we now come to the matter of the essential nature of a substance. A substance is essential if it is indispensable for the functioning of the organism and also cannot be replaced in its entirety by another substance.
A few substances seem to have a positive effect on an organism without being essential, so called beneficial elements. Here, however, refined methods of analysis and proof have in recent years led to many new discoveries.
Selenium, for example, is currently widely talked about as a trapper of free radicals in animal metabolism, but a mere 20 years ago only the toxicity of selenium in high doses was known.
The same applies to chromium, which in the most minute quantities appears to play a part in the so-called glucose tolerance factor (GTF): although the exact importance of chromium has by no means been clarified as yet.