Fire when plane enters area

I'd just use Vector3.Distance instead...

@HideInInspector
var player : GameObject ;
@HideInInspector
var _myTrans : Transform ;
var inRange : float = 100 ;
@HideInInspector
var distance : float ;
 
function Start(){
player = GameObject.FindWithTag("Player") ;
var _myTrans = transform ;
}
 
function Update(){
distance = Vector3.Distance(player.transform.position, _myTrans.position) ;
   if(distance < inRange){
      TankAttack() ;
   }
}
 
function TankAttack(){
   //whatever it is you have for the tank to shoot
}

...somethin like that anyway. This script would attach to your tank object.