A combined biofilter system including a trickling sand biofilter and an ulva biofiler (Ulva.pertusa) was set up for water quality control of fish tank. The water purification characteristics and the efficiency of the combined biofilter system were tested by nutrient supplying experiment in a recirculating abalone-culture system. The results showed that the combined biofilter system was effective in ammonia removal when ammonia concentration is less than 2mg·L^-1, and the removal rates of ammonia and dissolved organic in fish tank were more than 80% and 40%, respectively, and the pH and DO were also maintained at required levels. The developed combined biofilter system could provide one alternative for water quality control of fish tank.