// Assuming there are no more tanks that spawn during gameplay you can store them so you don't have to call FindGameObjectsWithTag all the time as it's a slow operation!

GameObject currentTarget;
List<GameObject> tanks;

void Start()
{
    tanks = new List<GameObject>(GameObject.FindGameObjectsWithTag("Tank"));
    tanks.Remove(this.gameObject);

    // Get closest tank
    GetClosestTank();
}

private void GetClosestTank()
{
    int closestTankIndex = 0;
    float closestDst = float.MaxValue;

    Vector3 myPos = transform.position;
    for (int i = 0; i < tanks.Count; i++)
    {
        // Check if the tank we're looking at is closer than the closest tank
        float dstToTank = (myPos - tanks[i].transform.position).sqrMagnitude; // sqrMagnitude is faster than Vector3.Distance
        if (dstToTank < closestDst)
        {
            // If it is the closest then update values
            closestDst = dstToTank;
            closestTankIndex = i;
        }
    }

    // Finally set currentTarget to the closest tank!
    currentTarget = tanks[closestTankIndex];
}