To enhance water quality in eutrophic urban ponds, the effects of a portable water circulation device, equipped with biological oxidation carriers and light-emitting diodes (LEDs), on dissolved oxygen (DO), water transparency, total nitrogen (TN), total phosphorus (TP), chemical oxygen demand (COD), and plankton community in laboratory and field mesocosms were examined. In the laboratory, water quality was monitored in vessels containing water and mud collected from an urban pond, which were subjected to different treatments (control, device without LEDs, and device with LEDs) for 28 days. In the treatments using the device (with and without LEDs), DO increased by aeration, whereas TP decreased by mitigation of mud anoxia. Reductions in TN and the presence of phyto- and zooplankton were observed in those treatments with LEDs, in which periphyton growth was stimulated in the carriers. Under field conditions, water quality of the pond was monitored for 40 days in enclosures. Clear increases in DO and transparency and reductions in TN, TP, and COD were detected in the treatments using the device. Reductions in cyanobacteria and increases in diatoms, green algae, and zooplankton were observed in the treatments with LEDs. Thus, the circulation device effectively improved oxygen condition of the bottom and nutrient flux between sediment and overlying water. In addition, these results suggest that the COD removal and growth of diatoms and green algae by LEDs in carriers play roles on developing periphyton at the bottom and suitable phytoplankton community in the water column.