A novice attempts...

One error that strikes me is the fallacy of affirming the consequent. It is an argument in the form m→n; n, therefore m. A Truth Table shows this argument is invalid. It does not give us reason to either believe or deny the conclusion. This argument gives us reason to believe absolutely nothing.

I think the real problem here is ambiguity. When we say "are made of," do we mean "primarily" or "exclusively" or just "partly"? Because fish are partly "made of" metal (we all are). So in that respect, this is not fallacious.

If by "are made of" we mean "primarily" or "exclusively," then we have a simple non sequitur . It doesn't follow that because something is high in a certain mineral that it is "primarily" or "exclusively" made of that mineral. It can also be argued that this is simply factually incorrect and there is still no fallacy.

A quick Google search has also turned up some debate on what actually constitutes a "metal." The explanation was a bit above my pay grade, but the argument was that the "metals" in our bodies are technically not metals given the form they are in. If that is the case, then we are dealing with the fallacy of composition . Consider:

A (iron) is part of B (fish).
A (iron) has property X (metal).
Therefore, B (fish) has property X (metal).

So the "novice" attempt at what might appear to be a simple fallacy is actually quite complex and debatable in terms of its fallaciousness.

fallacy of composition