Common co-existing organism Euplotes (Ciliate, Protozoa) well often observed in the rotifer mass culture tank of marine fish larval rearing stations. Ciliate protozoa (Euplotes) competed to condensed food (micoralgae, Nannochloropsis oculata) with rotifer (Brachionus rotundiformis). As results, rotifer density was rapidly decrease, and what is more, the rotifer was wiped out by dominant bacteria species at that time.