Here's a short extract from our user manual - the Product guide which every box with the device is supplied with. In case you're wondering if the reservoir and the water tank are the same thing: yes, they are.
DO NOT INSTALL WATER TANK WITH WATER WITHOUT CARTRIDGE INSIDE
Filling the reservoir
The device only needs water to work properly. You should remove the reservoir from the left side of the device, fill it with water and then place it back. The water will start flowing automatically.
Depending on the quality of the air in the room, the full reservoir should provide from 6 to 8 hours of uninterrupted cooling. To extend this time you need to fill up the reservoir, place it in the device, wait until the cartridge is full of water and then refill the reservoir. After this operaton, the running time will be extended by 1 to 3 hours. For instance, this may be useful if you want to use the device whilst you sleep. The transparent reservoir lets you assess the water level instantly. It is worth menthioning that the evaporative cartridge will absorb up to 400 ml of water which means that once the reservoir is empty, the device will continue cooling for another few hours. The default mode will have the device notify you by illuminating the reservoir in red when the water runs out. You can turn off these notifications at any time via the mobile application. When the water runs out completely, the device will continue working as a regular fan. You can use the replaceable reservoir whilst the device is working as it will not affect its operation.