ANSWER: What you have there, Bryce, is a rock flagtail, Kuhlia rupestris, also known in Australia as jungle perch, or throughout the Pacific Islands as river fish. This species is the largest member of the family Kuhliidae (flagtails), with females growing to more than 20 inches long. In contrast, males seldom exceed 12 inches. The rock flagtail is also the most widespread of the family, occurring throughout coastal rivers in the Indo-Pacific region, from the western Indian Ocean to Japan and northern Australia, and as far east as Fiji and Samoa.
Flagtails were introduced to Hawaii in the 1960s but failed to acclimate. Rock flagtail spend a significant portion of their lives in freshwater reaches of clear, fast-flowing coastal rivers and streams, but adult fish (over 8 inches) seasonally migrate into brackish estuary and river-mouth areas to spawn. Indeed, planktonic eggs and larvae of K. rupestris survive in salt water for around 40 days before metamorphosing into juvenile fish. This longer period in the plankton stage compared to other flagtails is thought to have allowed the species to utilize currents to island-hop across the western Pacific. Once back in their coastal streams, juvenile and adult rock flagtail are omnivorous, feeding on small fish, crustaceans, and even insects and fruit.
— Ben Diggles