This study investigated the effects of two different water sources, artificial seawater prepared with Red Sea Salt and natural brackish water from Kampot, on the water quality dynamics, growth performance, and survival of mud crab (Scylla serrata) cultured in a recirculating aquaculture system (RAS) under indoor conditions. The experiment was conducted over 60 days using a controlled setup where individual mud crabs were stocked separately to reduce stress and cannibalism. Key water quality parameters, including temperature, salinity, pH, dissolved oxygen (DO), ammonia, and nitrite, were monitored daily using calibrated instruments. Growth and survival performance were assessed at the end of the trial, and statistical differences between treatments were analyzed using one-way ANOVA. Results showed that both water sources maintained stable water quality, with salinity around 30 ppt and DO levels above 5 mg/L. Ammonia and nitrite remained at non-toxic levels due to proper system management. However, water temperatures frequently exceeded 35 °C, a threshold known to induce thermal stress in Scylla serrata. Crabs cultured in artificial seawater exhibited significantly higher final weight and weight gain (p < 0.05) compared to those reared in brackish water, indicating a more favorable environment for growth. Despite this, survival rates were relatively low in both treatments (45% and 40%), likely due to elevated temperatures affecting molting and immune function. These findings demonstrate that artificial seawater is a viable substitute for natural water in indoor mud crab farming and may enhance growth outcomes. However, the study also highlights the need for temperature control strategies to improve survival rates. This research contributes practical insights for optimizing water quality management in tropical aquaculture systems and supports the development of sustainable, high-efficiency indoor farming practices for mud crab.