Fish is a simple Legacy variant of the known Fish concept, combining small blue creatures with counterspells. It therefore falls into the Aggro-control deck category.
The deck usually plays a varied amount of Merfolk creatures with multiple Lords such as Lord of Atlantis and Merrow Reejerey. It uses Aether Vial to play more of these Merfolk while keeping mana open to counter spells the opponent plays, play additional creatures or additional supporting cards. Aether Vial also enables the player to put threats on the board without triggering Standstill, a card the deck plays to gain card advantage when the opponent is forced to play a card or being downed by creatures. The counterspells of Daze and Force of Will are used to prevent the opponents plays while being tapped out. Other supporting spells like Echoing Truth and Umezawa's Jitte are also used.
Some variations splash into other colors for cards such as Tarmogoyf.