Fish do not have penises. This is a specific reproductive structure that is only associated with tetrapod vertebrates (tldr, all vertebrates apart from fishes, i.e. mammals, birds, reptiles).

Most fishes have external fertilization, with the females releasing eggs into the water or onto some structure, and males releasing sperm onto the eggs to fertilize them. There are some groups of fishes which have internal fertilization. Elasmobranchs (sharks and rays) are the most obvious examples, with males possessing claspers. Claspers are calcified extentions of the pelvic fins that the males insert into the female's cloaca for internal fertilization. Claspers are often referred to as penises, however they are different in their form, orgin, and function. There are other types of fishes that have internal fertilization, however the mechanics behind that are not as well understood.