Trash fish is common feed for caged fish in marine aquaculture. Most feed is not eaten and enters the water surrounding fish farms. The organic matter in trash fish is a nutrient source contributing to aquatic eutrophication impacts such as algal blooms and low oxygen. The objective of this study was to examine whether phytoplankton utilized organic matter of trash fish directly. Fifteen microalgae species were cultured in the medium made of open oceanic water with f/2 enrichment (Treatment A, f/2 medium), with only silicate added (Treatment B, for diatom species) and with fish tissue+silicate (Treatment C). Four species grew significantly faster on fish tissue than in f/2 medium, another 4 species had similar growth rates between the two treatments (A and C). Growth rates of Pyramimonas sp. on different amount of fish tissue appeared to increase initially with increasing added fish tissue weight, reached a maximum at an intermediate amount of fish tissue additions. Time course of batch culture of Chaetoceros curvisetus showed that the species was capable of utilizing organic nitrogen released from fish tissue and grew well. Dissolved organic nitrogen (DON) was lower in the batch culture with algae and fish tissue than that with fish tissue but without algae in the medium, which indicated a direct utilization of DON during algal growth. These results indicated that fish tissue could be a direct nutrient source to support phytoplankton growth and coastal management needs to pay attention to organic contamination from trash fish.