How about favorite Moon Light shots, or Corals doing things out of the ordinary, or Corals feeding....how about a three picture montage of a coral in action or feeding.
I think you need to pick topics that are general and not restrictive to those that don't have the equipment (like actinic or moonlight shots). Perhaps just one word general themes
Examples:
Pairs
Long and short
Rainbows
Swaying in the wind
Food fight
Razor sharp
Make topics where people need to think about the compositions and can give different interpretations depending on what is in/around their tank. Just a humble thought.