The instruction to avoid red wine with fish is usually dismissed as arbitrary tradition. In one specific case it describes a real chemical reaction with a repeatable result.
The reaction involves iron
Wine contains small quantities of iron absorbed from soil, from equipment and from processing, and the amount varies considerably between wines and regions. It is present in every bottle at some level.
When iron encounters certain unsaturated fatty acids abundant in oily fish, it catalyses their oxidation. The products of that reaction taste distinctly metallic and fishy, and they are formed in the mouth rather than on the plate.
The sensation lingers well after swallowing, which is why the effect is so noticeable and so unpleasant when it occurs.
It is the iron level that predicts it
Wines with higher iron content produce the effect more strongly, and this correlates loosely with red wines but is not exclusive to them.
A white wine with elevated iron can produce the same result, and a low-iron red may not produce it at all.
This explains why the traditional rule has always had exceptions that its defenders struggled to account for.
Tannin and sulphur compounds add their own problems
Separately from the iron reaction, tannin binds to proteins in delicate fish and can produce a drying, chalky texture against soft flesh. The contrast is unflattering to both.
Some seafood carries sulphur compounds that clash with reduced characters in wine, producing aromas neither item shows alone. Shellfish are the most frequent source of that particular problem.
These effects are less dramatic than the metallic reaction but contribute to the general reputation of the pairing.
Preparation changes the outcome
Cooking method matters, since grilling and searing add browned, savoury compounds that bridge towards red wine effectively.
Accompanying ingredients do the same, and a fish dish built around mushrooms, lentils or a meat-based sauce shifts towards the wine rather than away from it.
Acidic elements in the dish also help, since acid moderates the perception of the metallic character when it does appear.
How to apply this in practice
The reliable version of the rule is narrow: oily fish with tannic, iron-rich red is the combination to avoid.
Light reds served slightly cool work well with a wide range of fish, and the tradition of doing so is long established in several regions.
Testing a small amount before committing the whole meal costs nothing, and the reaction announces itself immediately when it is going to happen.