Purun Tikus (Eleocharis dulcis L.) is a plant in the natural aquatic ecosystems of acid sulfate swamp; it has potential as a bio filtered metal pollutant. This plant can grow and adapt well to the flooded acid soils. In this study, Purun Tikus was planted in the Horizontal Subsurface Flow Constructed Wetland (HSSF-CW) system. This HSSF-CW system was designed using subsurface flow and take advantages of the natural processes that occur therein. The process in this wastewater system treatment is involving wetland vegetation, soil growing medium, and wetland microbial communities. The HSSF-CW with Purun Tikus was known able to decrease concentration of Fe in the AMD of coal. However, populations of plants and its seedlings in accordance with the HSSF-CW system is unknown. Therefore, this study was conducted to determine the space-planting and Purun Tikus seedlings in the HSSF-CW system which is able to decrease concentration of Fe in the coal AMD. This study used a model (reactor) of HSSF-CW with dimensions of 100 cm x 30 cm x 35 cm. Purun Tikus were planted with a spacing of 5 cm, 10 cm, 15 cm; and the seedlings are tiller, young plant, and old plant. Results showed that a space-planting and seedlings are significantly influencing the decrease of Fe concentration in the AMD coal. Decreased of Fe concentration was higher in the treatment of 15 cm planting space and the seedling is "tiller". KEYWORDS: Purun-tikus, Horizontal Subsurface Flow Constructed Wetland (HSSF-CW), iron, acid mine drainage.