BACKGROUND: Balance in the amount of minerals in the aquatic animal’s diet is very important. Phosphorus is one of the most important minerals in fish and aquatic animal’s nutrition. Phosphorus is important not only for the proper growth and fish health, but also for excessive phosphorus excretion that may cause water pollution and algal bloom. OBJECTIVE: In this study the effects of adding monoammonium phosphate in the diet of fish (Cyprinus carpio Koi) and its comparison with monocalcium phosphate were investigated. METHODS: 650 koi fish weighing 17 g were distributed in 15 tanks (5 treatments - each in three replicates) and mono-ammonium phosphate-fed diets at three levels (0.75, 1.5 and 3% of the diet), monocalcium phosphate in one level (3% of the diet) and a control group (no phosphorus supplementation) were fed for 8 weeks. RESULTS: Growth indices and feed conversion ratio in fish fed 3% monoammonium phosphate showed the best results. The control group showed the lowest weight gain and the highest feed conversion ratio. Blood phosphorus was also significantly correlated with increasing monoammonium phosphate. The highest level of calcium in the blood was observed in the group fed monocalcium phosphate (3%). CONCLUSIONS: Considering the improvement of growth and nutrition indices, adding 1.5 to 3% monoammonium phosphate in the diet of koi fish was suggested.