The answer to this question really depends on the way you're preparing your fish. If you've steamed, baked or sauteed your fish, you're in luck -- all you have to do is turn the heat off, put a lid on your pot or pan, and let it sit for up to 20 minutes. The steam will gently warm your fish until you're ready to serve. If you've taken care not to overcook your fish in the first place, keeping it steamy for 20 minutes won't dry it out.
But when it comes to retaining a crispy skin or batter on skin-on or fried fish, it gets a little trickier. The biggest cooking error that leads to sogginess is wrapping or covering anything with a crunchy coating. Though it's tradition in Britain to deliver fish and chips wrapped in newspaper, it's actually a huge mistake. The contents of the package end up steaming themselves into soggy submission.
To keep batter or skin crispy when you're cooking up fish in batches, try this technique:
Heat your oven to 200 degrees Fahrenheit.
When fish is thoroughly cooked and ready to keep warm, transfer it to a wire rack placed over a baking sheet. Do not cover or wrap in foil! Hold in the oven for up to 30 minutes.
When it's ready, eat it immediately, and eat it all -- especially if it's fried. Fried foods turn to mush as leftovers, so enjoy it in all its crispy glory.
* And when keeping food warm or letting it sit out at room temperature, always keep in mind the cooking "danger zone," of which few home cooks are aware. Food should never stay between 41-135 degrees Fahrenheit for more than four hours. At that temperature range, bacteria begins to grow rapidly and will quickly take over your food. (Keep this in mind for those summer picnics!)