Certron Corp. reported a loss of $54,000 for the fiscal third quarter ended July 31. It had net earnings of $23,000 for the year-earlier period. Net sales were $7.1 million, a 9% increase from the $6.5 million reported for the fiscal 1987 third quarter. The Anaheim magnetic tape maker said that although sales rose, the increase came from videocassette sales, which required increased promotion and ad spending because of highly competitive market conditions. Also contributing to the company’s loss was the bankruptcy of a major retail account, which resulted in bad debt and increased interest. For the nine months, net income was $122,000. It was $205,000 a year ago. Sales were $20.3 million, up slightly from the $20.2 million reported for the nine months last year. Certron manufactures audio recording tapes and floppy disks, distributes videocassettes, and manufactures and assembles products for other companies.