What main quality should be avoided when pairing bitter beers with oily fish?

When pairing bitter beers with oily fish, avoiding metallic flavors is crucial because these flavors can clash unpleasantly with the rich, fatty characteristics of oily fish. Bitter beers often have strong hop profiles, which can enhance their bitterness and potentially bring out any metallic notes present in the beverage. When these two elements interact, it can result in an off-putting taste that overwhelms both the beer and the food, leading to an unbalanced pairing.

In contrast, while sweetness, fruitiness, and herbal notes can have varying effects in different pairings, they do not generally carry the same risk of creating a disruptive taste experience when matched with oily fish. Sweetness may even complement the bitterness of the beer, providing a counterbalance, while fruitiness and herbal notes can add complexity and freshness. Thus, metallic flavors are particularly detrimental in this context, making them the main quality to avoid.