Problem instantiating projectiles

I am getting the error, Error CS0030: Cannot convert type 'UnityEngine.GameObject' to 'Fireball'

The Aoe line works fine but so I could technically use that for the fireballs but I don't know how to propel the fireballs forward. I would like to learn why I am getting the error if anyone knows the solution.

 using UnityEngine;
 using System.Collections;
 using System;
 using System.Collections.Generic;
 
 public class Staff : MonoBehaviour, IWeapon, IProjectileWeapon, IAoe 
 {
     [PunRPC]
 
     private Animator animator;
     public List<BaseStat> Stats { get; set; }
 
     public Transform AoeSpawn { get; set; }    
     Aoe fireAoe;
 
     public Transform ProjectileSpawn { get; set; }
     Fireball fireball;
 
     void Start()
     {
         fireball = Resources.Load<Fireball> ("Weapons/Projectiles/fireball");
         fireAoe = Resources.Load<Aoe> ("Weapons/Projectiles/fireAoe");
         animator = GetComponent<Animator> ();
 
         }
         
     public void PerformAttack()
     {
         animator.SetTrigger ("Base_Attack");
         //Debug.Log (this.name + "Sword Attack!");
         }
     public void PerformSpecialAttack()    
     {
         animator.SetTrigger ("Special_Attack");
         //Debug.Log (this.name + "Special Sword Attack!");
     }
 
     public void CastProjectile()
     {
 
         //PhotonNetwork.Instantiate ("fireball", ProjectileSpawn.position, ProjectileSpawn.rotation,0);
 
         Fireball fireballInstance = (Fireball)PhotonNetwork.Instantiate ("fireball", ProjectileSpawn.position, ProjectileSpawn.rotation,0);
         fireballInstance.Direction = ProjectileSpawn.forward;
 
     }
 
     public void CastAoe()
     {
         PhotonNetwork.Instantiate ("fireAoe", AoeSpawn.position, Quaternion.Euler(270,0,0),0);
     }
 
 }


 public void CastProjectile()
     {
 
         //PhotonNetwork.Instantiate ("fireball", ProjectileSpawn.position, ProjectileSpawn.rotation,0);
 
         Fireball fireballInstance = (Fireball)PhotonNetwork.Instantiate ("fireball", ProjectileSpawn.position, ProjectileSpawn.rotation,0);
         fireballInstance.Direction = ProjectileSpawn.forward;
 
     }