Terminalia catappa leaf, also known as Ketapang leaf, is well known for its medicinal properties and long history of use in aquaculture. Studies on the effects of leaf extracts on fish productivity are scarce. This study performed a time-series observation of ground young and mature T. catapppa leaves on tropical phytoplankton productivity. The young and mature leaves were prepared as fine powder separately. For the time-series experiment, 10 grams of each ground leaf were added to 10 L of lake water containing phytoplankton and incubated at room temperature with ambient illumination. The chlorophyll a saturation optical density was quantified on alternate days using a spectrophotometer and the data were analysed using Origin 6.0 Software. On Day 11, the chlorophyll a percentage had decreased by 73% and 81% in the control and mature leave treatment carboys, respectively. Meanwhile, the sample treated with young leaves of T. catappa showed only a 29% decrease. The results have shown that phytoplankton treated with young T. catappa powder demonstrated a positive but weak correlation (R² = 0.123), represented by the chlorophyll saturations. This study supported the hypothesis that young T. catappa leaves sustained phytoplankton growth.