He took a photo of the unknown specimen, released it back into the water and then sent the picture to Mote Marine.
They quickly informed Rigby that he had caught a pacu, a member of the piranha family.
"I'm looking at all of the teeth, saying this thing can do some serious damage to another fish or something," Rigby told WWSB.
Rigby was right to be nervous about the potential damage the pacu's mouth could cause.
"I found out it has a reputation for going after men's testicles. I was just worried about it biting my finger," he said.
The fish is a native of South America, but it's possible that someone released it into the water after keeping it as a pet.