What kind of fish do you have? (question implies that you either have only one fish, or more than one fish of the same kind)
What kinds of fish do you have? (implies that you have more than one kind of fish)
"Fish" and "fishes" can both serve as the plural of fish(s) and there is no difference in meaning between them whatsoever, although fish would probably be the more usual choice for the plural in contemporary English.
Chocolates are small individual portions of chocolate, around about one mouthful in size, sometimes with fillings, and often sold together in a pack within which there are often various kinds.
Chocolate is a bigger lump of the same material often sold in individually wrapped portions.