Myth: red with meat, white with fish.
Ignore this myth. The theory comes from the fact that meat protein can help soften tannins when eating, but evidence suggests that salt is the more important factor here. Many great red wines go with lots of fish dishes.
The only hard and fast ‘avoid’ rule I’d suggest, is oily fish & red wine. Together these produce a horrible metallic taste in your mouth that can ruin both the food & the wine. So next time you’re diving into a kedgeree, grab a white and not the red.
Umami is the other flavour to consider. This makes tannins more astringent, so red wines clash quite badly with it. This isn’t limited to fish, however, and is true across meats, vegetables and sauces too.