So to change the rate of flow you'll need to figure out what your model's unit measurements are. So if your model is in liters, just set the tank's maximum content to 12,000 then just figure the rate of flow needed so that it fills 12,000 over the course a day period based on your time units.
To make sure the tanks don't empty until they are full, just set an On Reset trigger that closes the outgoing port of the tank, and then set an On Full trigger to open the port once it's full.
