Rehabilitation under water is a viable physical rehabilitation option, but it has some limitations in terms of adapting to needs of each patient. In addition, its facility requirements are relatively high. Simulating the fluid environment using a robotic system would enable therapists adjust various parameters so that the therapy is tailored to the patient's unique state. Also, using a robotic system instead might be less costly and easily reachable. In this study, human lower extremity movement in fluids is modeled. This model is verified by comparing computer simulations with the results of previous experimental studies. Then, the model is used to create a control scheme which is implemented on a robotic gait trainer. Output torques are measured to check the effectiveness of the controller in simulating the fluid environment while compensating for weight and friction of the robotic system. Measurements showed that the desired joint torques were achieved and the controller was able to make the orthosis transparent to the patient. A hip extension exercise used in aquatic therapy was performed with the robotic system while varying drag coefficient, fluid density and flow velocity, and the data collected is presented.