In aquaculture, determining the production process of metabolites and changes in water quality and biodegradation according to the cultured species plays an important role in determining the capacity of the cultured systems and how to set the appropriate environmental conditions for the cultured species. Therefore, Therefore, in this study, in a completely randomized design, rainbow trout (Oncorhynccus mykiss) in four groups with an average weight of 20±3, 5±50, 9±100 and 10±200 g (SD±mean) were introduced to 300 liter fiberglass tanks (200 liters) as experimental treatments with three replications and fed commercial food for three days. Then the fishes were separated from the tanks and the biodegradation process of their metabolites was investigated. When the fish were present in the tanks and the ammonia was excreted from them, the concentration of ammonia in the water increased significantly until the third day of the experiment, but after the fish were collected, the ammonia excretion stopped. Following the nitrification phenomenon, the concentration of nitrite and then the nitrate increased and the nitrate accumulated in the water and its concentration increased over the time.  In the continuation of the experiment, due to the phenomenon of nitrification, the concentration of nitrite and nitrate increased. The phosphorus content of water (during the experiment) was significantly different among treatments (except the first and fourth weights) (P<0.05). An increase in electrical conductivity (between the starting and third days) was completely evident during the first three days of the experiment, but after the collection of fish, its increase continued with a lower intensity.  Significant difference was observed in total hardness value among different fish weights (P<0.05). Other elements such as sulfate, calcium, magnesium, potassium, sodium, iron, manganese, molybdate, and total hardness were also measured. However, some of them were produced in relatively small quantities by fish, or were insignificant in the diet. For this reason, their concentration in water was negligible, or their changes did not follow a specific law over time. The results of this study showed that, it is possible to measure the amount of nutrients produced in the consumed food unit and to maintain the water quality, the waste can be removed from the water as soon as possible so that it does not increase the concentration of pollutants in the water. According to the rates of food consumetion, temperature of the water and changes in water quality, it is possible to estimate the rate of biodegradation of organic matter and metabolite production in a culture system of rainbow trout.