My ideas:
1) Guess the fish
2) Lose your letters : it's a game where you have all the letters of the Alphabet apart from X on a set of cards, the cards are dealt out evenly between the players and a topic is given, for example 'Tetras'. The player with 'N' shouts Neon, the one with 'C' says Congo, etc..The last player to put down their letter in the center of the table (it's a game of speed) has to pick up all the letters everyone else has put down. The winner is the first player with no letters. Could be fun.
3) Labyrinth: A game where you move players around the board to get to different places... replace the ghosts & knights with fish & plants?
4) Poker with fish on the cards
Keep us posted on what you choose!