Error: Onaccount111 Compile Error: Incompatible types since an instance of SObject is never an instance of Account at line 30 column 13

VF Page :

<apex:page standardController="Account" extensions="Onaccount111" sidebar="true" id="D001">
<apex:sectionHeader title="Account" id="S001"/>
<apex:form >
<apex:pageBlock id="pb001">
<apex:pageBlockButtons location="top">
<apex:commandButton value="Open"/>
<apex:commandButton value="Delete"/>
</apex:pageBlockButtons>
<apex:pageBlockSection id="pbs" columns="1" title="Search Leads">
<apex:inputField id="f0" label="Parent ParentAccount" value="{!Account.Parentid}"/>
<apex:inputField id="f1" label="Filter by Accountname" value="{!Account.Name}" required="false"/>
<apex:inputField id="f2" label="Filter by Type" value="{!Account.Type}"/>  
<apex:inputField id="f4" value="{!Account.Active__c}" label="Active"/>
<apex:inputfield id="f5" label="Priority" value="{!Account.CustomerPriority__c}"/>
</apex:pageBlockSection>
</apex:pageBlock>
</apex:form>
</apex:page>



CLASS :

public class Onaccount111
{
    public list<account> acclist{get;set;}
    public Account acc{get;set;}
    public string Accountname{get;set;}
    public boolean pageblk{get;set;}
  //public string parentaccount{get;set}
    public string type{get;set;}
    public string active{get;set;}
   // public integer start{get;set;}  
    public boolean previous{get;set;}
    public boolean next{get;set;}
    public boolean first{get;set;}
    public boolean last{get;set;}
    public string query{get;set;}
    public string priority{get;set;}
    public string accstatus{get;set;}
    public string dispstatus{get;set;}
    public string openstatus{get;set;}
       public string download;
    public string start{get;set;}
    public string finalsort = '';
    public string ss1 = '';   
    public string reqstatus;
    public map<string,string> fieldmap = new map<string,string>();

    public Onaccount111(ApexPages.StandardController controller)
    {
        acc = new Account();
        acc=(Account)controller.getrecord();
        acclist= new list<account>();
        //start=0;
        dispstatus='open';
        openstatus='open';
       // first='true';
       // next='false';
       // previous='true';
       // last='false';
        pageblk =false;
         download = ApexPages.currentPage().getParameters().get('download');
        fieldmap.put('Name','Name');
        fieldmap.put('AccountNumber','AccountNumber');
        fieldmap.put('Owner','Owner');                            
        fieldmap.put('Parent','Parent');
        fieldmap.put('Active','Active__c');
        fieldmap.put('CustomerPriority','CustomerPriority__c');
    }
    
    public void display(string status)
    {
         
        pageblk=true;
        this.parentaccount=acc.Parent;
        this.Accountname=acc.Name;
        this.type=acc.Type;
       this.priority=acc.CustomerPriority__c;
       this.active=acc.Active__c;
       this.accstatus=status;
      this.sortby=acc.Sort_By__c;
       list<string> sortby1= new list<string>();
        
       /*** if(sortby != null)
        {
            sortby1=sortby.split(';');
            integer size= sortby1.size();

            for(string ss: sortby1)
            {
                string search = ss;   
                    if(fieldmap.containskey(search) == true)
                    {
                        ss1 = fieldmap.get(search);
                    }
    
                    if(Ss != sortby1[size-1])
                     {
                        finalsort = finalsort + ss1 + ',';
                     }
                    if(Ss == sortby1[size-1])
                    {
                        finalsort = finalsort + ss1;
                    }
            }        
        }***/
        
        if(status == 'allaccounts')
        {
            //if(sortby1.size() == 0)
           
                query = 'select id,status__c,type__c,name__C,Created_Date__c,Lead_Type__c,Status,Name,LeadAddress__c,Email,Phone,LeadSource,GTM_Appt_Schedule__c,Appt_from__c,Appt_Held__c,qbdialer__LastCallTime__c,pi__score__c,pi__first_activity__c,pi__last_activity__c,Previous_Pardot_Score__c,Pardot_score_change_date__c,qbdialer__Dials__c,Follow_up_Date__c,Trend_of_Pardot_score__c from lead where status=:openstatus';
           
         }
         }
        
    public void open()
     {
       validate();
        dispstatus ='Open';
        reqstatus='Open';
        display(reqstatus);
     }

  public void validate()
     {
        if(acc.Ownerid != null)
        {
             ApexPages.Message msg = new Apexpages.Message(ApexPages.Severity.Warning,'No Record Found' );
             ApexPages.addmessage(msg);
        }
     }
}

I have tried a lot but still cant get this. Can anyone tell why this error is coming?

Thanks in advance :)