Cobalt in fertilizer?

I've noticed some people suggesting there be some cobalt available for 
aquatic plants.

Is this really the case and if so what levels are optimal and what levels 
are toxic? What are cobalt deficiency symptoms if any? I was always under 
the impression that cobalt is more toxic than copper, but then again I'm 
not a chemistry student. 

James Wong 
u2120863 at acsusun_acsu.unsw.edu.au