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Until the user is no longer active, all Fire-type attacks used by any active Pokemon have their power reduced to 0.33x. Fails if this move is already in effect. This effect is not stackable.
This move, along with its Ground-type counterpart, Mud Sport, is fairly useless; most Pokemon that have access to Water Sport already resist Fire, and the ones that don't can just run Rain Dance if they want to replicate the effect, but also boost their STAB moves as well.
Mud Sport is a Ground-type equivalent.