Yes, there is fish hentai swimming around on the internet - Salmon hentai to be exact. I'm not surprised, it was just a matter of time (I'm just glad it's just fish. I don't think I'd be able to handle cephalopods).
 
The fairly brief story follows the seduction of a student fish by his (obviously sexy) tutor fish. Explicit fish frolicking ensues. Very hot.
 
If the above description (or the title image for that matter) wasn't repugnant enough to scare you away, read the rest here .
 
Via Japanator