Inventory stack system problam

So i've made an Inventory with a stack system but when I add stacks if stacks are added to every items in the inventory instead of the one I want to add to they all get the same amount of stacks

     public void AddItem(int id, float stacks)
     {
         for (int i = 0; i < inventory.Count; i++)
         {
             if(inventory[i].itemName == null)
             {
                 for(int j = 0; j < database.items.Count; j++)
                 {
                     if(database.items[j].itemID ==id)
                     {
                         if(!InventoryContains(id))
                         {
                             inventory[i] = database.items[j];
                         }
                         else
                         {
                             AddStacks(id,stacks);
                         }
                     }
                 }
                 break;
             }
         }
     }
 
     public void AddStacks (int id,float stacks)
     {
         for (int i = 0; i < inventory.Count; i++) 
         {
             for (int j = 0; j < database.items.Count; j++)
             {
                 if(database.items[j].itemID == id)
                 {
                     inventory[i].itemStacks = stacks + inventory[i].itemStacks;
                 }
             }
         }
     }
 
     public void RemoveStacks (int id, float stacks)
     {
         for (int i = 0; i< inventory.Count; i++)
         {
             if(inventory[i].itemID == id)
             {
                 if(inventory[i].itemStacks > stacks||inventory[i].itemStacks > 1)
                 {
                     inventory[i].itemStacks = inventory[i].itemStacks - stacks;
                 }
                 else
                 {
                     RemoveItem(id);
                 }
             }
         }
     }
     
     public void RemoveItem(int id)
     {
         for(int i = 0; i < inventory.Count; i++)
         {
             if (inventory[i].itemID == id)
             {
                 inventory[i] = new Item();
                 break;
             }
         }
     }
     
     public bool InventoryContains (int id)
     {
         bool result = false;
         for (int i = 0; i < inventory.Count; i++)
         {
             result = inventory[i].itemID == id;
             if (result)
             {
                 break;
             }
         }
         return result;
     }
 
     public bool InventoryContainsStacks (int id, float stacks)
     {
         bool result1 = false;
         bool result2 = false;
         for (int i = 0; i < inventory.Count; i++)
         {
             result1 = inventory[i].itemID == id;
             result2 = inventory[i].itemStacks == stacks;
             if (result1&&result2)
             {
                 break;
             }
         }
         return result1;
         return result2;
     }