How about a video submission?
Each contestant will enter a video they have filmed (or had someone help them film) that is of their fish - and tell a story.. such as a movie.
Can edit the video to add in effects, sounds, voices, cut scenes, etc.. but it has to have your fish as the main characters.
You can do an "Avengers" movie with your fish, get them moving around, looking silly, etc.. then can make your own voices or use the track of the movie and make the scenes.
I would say a 3-5 minute video would be good - not too long, nor too short. Can use original ideas or make a spin off of a favorite show/movie.
Imagine a "How I met your Mother" spin off of the fish meeting on the store shelves in cups, etc etc..
Only requirement would be that you have to have your fish in it being the main characters, you can be in it, but as a side kick or other smaller part.
Having editing done doesn't make it better - it's who makes you laugh/cry/smile the most.. so anyone who has access to a camera can do it. If you can't make a video (most digital cameras now have video options), you can make a snap shot video - pictures edited together.. think like a comic book.. with thought bubbles, etc.. but make it flashy enough that it can compete with a video.
Sorry if it's not described well.. I'm just getting over being sick, so my brain isn't functioning properly lol.