This is actually a good question, since many doctors exist for reptiles for example and other semi-aquatic animals (or even fully aquatic animals), but there aren’t vets that just specialize in fish for example. In many cases, some vets will take fish in (think of those goldfish or koi that live more than 10 years).
As far as actual locations and names of those vets, it might just be best to call into various semi-aquatic vets and see if they will handle aquatic species (and then mention fish and the fish species). However, a vast majority of research and help can be found online in communities like ours, and surgery is extremely rare with fish due to their high sensitivity and the possibly price tag with any surgery on any animal.