Obviously water goes in here, also filters or de-scaling tablets, but never both
at the same time.
Always fill the tank by removing it and taking
it to the tap. If you pour water into the tank from a jug while it is on the machine some water
will always splash out and get into the grinder or pre-ground chute.
The tank itself can be washed in soapy water when required.
If your machine cannot recognize that the tank is full and keeps telling you to fill the tank even
when it is full, the problem can be that the floating magnet in the tank is stuck. The magnet is in
a groove on the side of the tank facing the machine and is free to rise a short distance when the
tank is filled which triggers a sensor. Washing the tank will normally cure this problem. The magnet can be seen in its groove in the picture.
Some newer models do not have a magnet in the tank but use an electronic sensor, if this fails the only cure is to replace it.