A few genera of Pomacentridae exhibit a behavioral trait where they protect and maintain their algae garden territories on coral reefs. This is a key behavior for the target species, Stegastes nigricans. The existence of these territories is important as a source of food and breeding site. Recent studies have reported that environmental changes, such as ocean acidification (OA), affect the aerobic performance of fish, which may in turn affect their behavior which includes activity levels. This study investigates the changes in the weeding behavior of S. nigricans under future ocean acidification stress. Target species were caught and kept in experimental tanks for 32 days at two different pH treatments (Control: 8.2 ± 0.01; low pH: 7.7 ± 0.01). Their daily activities were videotaped and we recorded the daily weeding frequency on algae patch and tank walls, which also included the number of times they removed algae and other invertebrates from their territory. Since the ambient seawater temperature fluctuated during the experiment, two-way ANOVA tests were performed to observe the effect of both pH treatment and temperature change. The presence of this species and their weeding behavior plays a role in shaping coral reef ecosystems. But the ongoing rapid climate change may impact their livelihood and population in the future.