Here is the full list of all 11 Fish Duel achievements.
Eat 200 units of grass
Eat 200 units of meat
Kill your first opponent
Kill 50 opponents
Kill 1,000 opponents
Collect your first coin in the ocean
Buy an emote
Collect 100 coins
Buy all emotes
Eat 5,000 units of grass
Eat 5,000 units of meat