Water quality assessment and fish diet composition were employed to quantify the ecosystem changes within an oil palm landscape in northern Sarawak, Malaysian Borneo, focusing on feeding guild plasticity. The study aimed to assess fish food availability and dietary shifts as indicators of functional responses across varying levels of habitat transformation. Three plantation management practices were examined: Buffer zones (native forest buffers), modified buffer zones (reduced native forest buffers) and drainage systems. Clarias leiacanthus and Helostoma temminckii were selected due to their high occurrence and occupation of benthic and pelagic niches, respectively. The buffer maintained the highest-quality aquatic environments (water quality index [WQI] = 61.35; slightly polluted; Class III), with a decrease in the modified buffer (WQI = 58.84; polluted; Class III) and drainage systems (WQI = 48.85; polluted; Class IV). Dissolved oxygen was significantly reduced at drainage (1.73 ± 0.45 mg/L) compared to the buffer (4.30 ± 0.39 mg/L) (Dunn’s test, p < 0.001). Dietary analysis (frequency of occurrence) revealed both species maintained natural feeding patterns in buffer, while at drainage shifted from high-energy prey (fish, zooplankton) to low-energy resources (debris, plant matter). Shifting demonstrates feeding plasticity in response to reduced habitat quality and altered food availability. These findings highlight the ecological importance of maintaining native forest buffers as natural filters that sustain water quality and trophic structure, mitigating agricultural land-use impacts on peatland habitats.