Improve a select query with MINUS

In follwoing two tables cas.cass_siebel@cas_lnk and CASS_HISTORY, there are about 500,000 records in each table. The following query returns the records in cas.cass_siebel@cas_lnk, but it will not return the records with exact match in CASS_HISTORY. This query is quite slow. I wonder if there is way to speed up this query. For example, adding some more indexes in both tables is helpful or not.Any suggestions? Thanks.
SELECT casss.record_type,
       casss.fiscal_year,
       casss.sequence_number, 
       casss.office_number,
       substr(trim(casss.file_name), 1, 40) "FILE_NAME",
       substr(trim(casss.consumer_name), 1, 35) "CONSUMER_NAME",
       FNC_GET_COUNTY_CODE(casss.county_code) "COUNTY_CODE",  /* county_code is county_name in cas.cass_siebel */
       substr(trim(casss.insured_name), 1, 40) "INSURED_NAME",
       decode(length(trim(casss.company_code)), 5, trim(casss.company_code), NULL) "COMPANY_CODE",
       substr(trim(casss.company_name), 1, 40) "COMPANY_NAME",
       substr(trim(casss.agent_name), 1, 40) "AGENT_NAME",
       substr(trim(casss.agent_ssn), 1, 9) "AGENT_SSN",
       substr(trim(casss.agency_name), 1, 40) "AGENCY_NAME",
       to_date(casss.date_closed, 'YYYY-MM-DD') "DATE_CLOSED",
       to_date(casss.date_opened, 'YYYY-MM-DD') "DATE_OPENED",
       DECODE(casss.date_closed, NULL, 'Open', 'Closed') "STATUS",
       casss.origin_code,
       casss.COMPLAINANT_TYPE_CODE "COMPLAINT_TYPE_CODE", 
       casss.general_coverage_code,
       casss.detail_coverage_code,
       casss.reason_code,
       casss.disposition_code,
       casss.service_request_employee,
       substr(trim(casss.category), 1, 2) "CATEGORY",
       NULL "REFERRAL_TEXT",
       NULL "REASON_TEXT",
       NULL "referral_code_a",
       NULL "referral_code_b",
       NULL "referral_num_a",
       NULL "referral_num_b",
       to_date(casss.date_added, 'YYYY-MM-DD') "DATE_ADDED",
casss.company_complaint_ind "COMPANY_COMPLAINT_INDICATOR"
FROM   cas.cass_siebel@cas_lnk casss
WHERE  casss.date_opened IS NOT NULLMINUSSELECT CH.* FROM CASS_HISTORY CH