A filter adaptor assembly for a water cooler of the type having a support housing provided with a water bottle support at a top end to support an inverted water bottle over a water reservoir of the housing. The filter adaptor assembly comprises a support platform having a support base and it is connected in support engagement above the water reservoir. Supports are connected to the support base for securing one or more filter cartridges thereto. A dispensing valve is secured under the base and has a conduit for receiving treated water from the filter assembly. The valve has a water level sensing float mechanism which is adapted to detect the water level in the reservoir to dispense or cut-off the supply of treated water to the reservoir. The filter assembly is also connected to a pressurized water supply.