Query on design of domestic water tanks in apartments - How to link additional raw water tank to the existing structure?

Hello,

I have a general query on water tank design in apartments.
We have the following components in a water tank:
1) fire tank
2) raw water tank
3) softener tank

As per my knowledge, the flow must be the following: When full, the water flows from the fire tank into the raw water tank. The softener tank is for softening the water from the raw water tank. I have the following questions:
Q1) How should the flow be?
Q2) If we are constructing additional raw water tank space, what is the best way to link to the old tank?

Regards,

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