Donald Earl Fite III, 27, pleaded guilty Tuesday to animal abuse and domestic violence assault.
According to court records, Sarah Harris had broken up with Fite but returned to her Portland apartment July 25 to find him lying on her bed, saying he wanted to get back together.
When Harris tried to leave, Fite shoved her against a wall, grabbed her hair and tossed her against a bathtub.
Harris fled but returned with an officer to find her fish — a bright purple betta named "DeLorean" — on the wood floor with a knife through it.
According to court records, Fite quickly admitted killing the fish, telling police: "If she can't have me, then she can't have the fish."
Fite's attorney, Tom Macnair, said Tuesday that killing the fish was a "very low point" in his client's life.
"He is absolutely mortified and ashamed about what he did to the fish," MacNair said in Multnomah County Circuit Court. Fite, who had no previous criminal history, declined to make a statement.
Harris did not attend the hearing. In a phone interview with The Oregonian newspaper, she recalled crying hysterically when she saw the fish with a knife sticking through it.
"Donald bought the fish for me, and I'm sure he knew how much I cared for it," she said.
Betta fish, also known as Siamese fighting fish, are colorful freshwater aquarium fish that usually are between 2 and 3 inches long.
Prosecutor Eric Zimmerman told Judge Eric Bergstrom that Harris plans to get a memorial tattoo of the fish and wanted Fite to pay for it. Bergstrom, however, declined to make Fite pay restitution for the tattoo.
The judge ordered Fite to stay away from Harris, but said he could have contact with fish.