Kim Zolciak and Kroy Biermann Had Blowout Fight Before He Filed for Divorce

Kim Zolciak and Kroy Biermann seemed like a couple on the mend from the outside -- but were fighting nonstop behind closed doors -- before one fight sent Kroy over the edge, causing him to file for divorce once again.

Sources close to the estranged couple tell us ... Kim and Kroy had been arguing for several weeks about money before Thursday's filing ... and things had gotten "nasty." As for specifics ... we're told each has been on the other's case about not paying various bills. They blame each other for their serious financial mess. As one source said, "They've gotten ridiculously petty over who hasn't paid bills. They're counting down to pennies."

We're told Kroy was regularly threatening to file for divorce. However, it was during a fight Thursday morning when Kroy threatened once again to file for divorce, Kim dared him ... so he did.

TMZ broke the story ... Kim and Kroy filed separate divorce petitions back in May,but had a change of heart in July -- pulling their divorce filings.

At the time, we were told they still really loved each other and could work things out -- leaning on religion to help get them through -- but clearly, things soured.

Kroy's asking for sole legal and physical of their 4 minor children as well as child support, alimony and attorney's fees.