Match your recipe’s method first: choose a firm-and-mild fish for pan-searing; choose a tender flaker for baking or broiling.
Use the 1:1 ratio by weight (or volume of fillet thickness). Keep fillets similarly thick so cook time stays close.
Adjust for leanness: if your substitute is very lean (cod/haddock), brush with oil or add a finishing pat of butter.
Cook by visual doneness: remove the fish when the center turns opaque and flakes easily with a fork—don’t wait for total dryness.
Taste and rebalance: if the substitute feels bland, add a squeeze of citrus or a pinch of salt to bring out the mild sweetness.