Water budget of the Columbus Active Thermal Control System (ATCS) is kept under control by monitoring the water amount stored inside the accumulators of the two WaterPump Assemblies (WPAs) and planning properly all the on-orbit activities requiring water removal from the loop like water sampling and OPA treatment or water refurbishment via NASA Fluid Service Systems (FSS). After more than 4 year of activity a new approach enriched the fluid management capabilities by running water transfer from the accumulator of the back-up unit WPA2 to the accumulator of the nominal pump WPA1, without stopping the water flow through the circuit. This paper describes the analysis run with ThermalHydraulic Mathematical Model (THMM) to define the transfer activity parameters, operations impacts, on-orbit results and the relevant lessons learnt on THMM and procedures optimization