A sweet and sour sauce goes especially well with deep-fried food, not just because it whets one’s appetite but, more important, because it counteracts any trace of grease. Such is, indeed, the case with fish. There are regional variations and personal preferences, but mainly a sweet and sour sauce is a mixture of vinegar and sugar, balanced by salt, and made more interesting by the addition of other condiments. Try this one, and then concoct your own.