Ornamental fish has been raising and as a favorite hoppy for Vietnamese people along time ago. Being one of the most popular aquarium fish, Koi fish is generally cultured in glass or cement tanks and earth ponds. Nowadays, several crucial points should be mentioned in Koi fish culture are water pollution, disease outbreaks, poor growth and slight pigmentation on fish body. This study focus on determination of an optimal ratio of plant protein source (soybean meal) which used to replace with fish meal in the Koi diet to minimize the level of water pollution and improve the growth performance of fish. Three experimental diets comprised of CT1, CT2 and CT3 with soybean meal rate in the diets ranged from 28.5- 44.5 percent and designed to meet nutritional requirements of Koi carp juveniles. The control treatment is a commercial feed which has been commonly sold on the market. After 3 months of culture, the optimal diet was CT3 which contained 36.23 percent protein, 7.93 percent lipid, 4.23 Kcalg energy, 44.5 percent soybean meal ration. Fish fed CT3 had a significantly difference (p0.05) in water quality and growth performance of fish compared to the others, correspondingly with the mean values of ammonia and nitrate content were 0.040 mg/l NH3-N, 0.27 mg/l NO2, weight gain (WG) 121.80 percent
specific growth rate (SGR) 0.95 (percent/day) and feed conversion ratio (FCR) was 1.6. Moreover, Koi fish fed commercial feed had poor growth, high FCR and low quality of water.