So it seems that researchers in Japan have determined the cause of that horrible metallic super-fishy taste that occurs when some red wines are paired with fish. It's iron. Specifically, the amount of ferrous ion present in the wine. You can read all about this discovery here in the Journal of Agricultural and Food Chemistry.
Admittedly, you'll have to be willing to plow through sentences like "Metal ions were analyzed by a postcolumn reaction with 4-(2-pyridylazo)resorcinol reagent combined with spectrophotometric detection," and "Total phenolics of wines were estimated according to the Folin-Ciocalteu method expressed as gallic acid equivalents," but what the heck, it's Wednesday afternoon and you're probably bored at work anyway, right?
If you do manage to wade through the article, you'll hit the payoff which is that tannins—long the scapegoat of bad fish-and-red-wine pairings—are entirely innocent. Yes, tannins are the Dreyfus in this whole fishy affair; blame not the tannins, friends. Instead, stick the onus on iron. Of course, there is one small hitch. As Mssrs. Tamura, Taniguchi, Suzuki, Okubo, Takata and Konno put it with appealing delicacy, "In daily life, it is difficult to predict the iron content in a bottled wine without opening it."
And, one might add, without subjecting it to a postcolumn reaction combined with spectrophotometric detection—but then, who doesn't do that sort of thing, these days?