Does anybody know how much collant (ltr.) is inside the tank?

3.3 litres total in system and 275 ml between top and lower level marks in tank, ... Marty

Thanks...

It will be very hard put all 3,3 litres of coolant into the circuit, because some coolant will remain into the circuit.
Howewer, you have to unscrew the tank from the frame and raise it to put the water into the circuit.
In fact, the tank is not the higher point of the circuit, so if you don't raise it you will not manage to get the water into the circuit.
Ciao