The biggest mistake people tend to make is to overcook fish. Overcooking spoils the flavour and texture of fish.
Seal on a high heat and then moderate heat to cook through.
As a guide fillets usually take 5-8 minutes to cook depending on their thickness.
When cooked the flesh appears white.
Fish continues to cook even after removed from the heat.
To test, insert a fork into the thickest part of the flesh and gently divide. It’s cooked if it flakes easily. With whole fish the flesh should fall off the backbone.