Usually a bad water pump leaks coolant from the weep hole just below the pulley. This is the number one indicator of water pump failure. Otherwise, if it is overheating, that may also be a cause if the fins are worn out.
Look on the ribbed side of the belt as it is going over pulleys. If you see cracks it would be wise to replace it.
You just replace it at the interval listed in your owner's manual or every 50,000 miles. If you are having problems and suspect the fuel filter is clogged, just replace it.
Know whether to fix or replace. Understand the efficiency math. Check the ducts. Lower your heating needs.
We do not know what you mean by "PFD", please do not use abbreviations. But the answer would be when the instructions for the "PFD" say it needs to be replaced.
I just want to know when do i have to replace the water tank.
It would greatly help to know what engine you have and if it has air conditioning (working or not)
You will not know. Just replace it every 50,000 miles.
You do not wait for signs it needs replacing, you just replace it at the mileage listed in your owners manual. No owners manual then replace it every 75,000 miles.
If the water is dripping, I would replace it just to be safe. It doesn't cost much money.
if you know the specific heat of the liquid you're using to replace the water, any liquid you want
When the pressure becomes erratic time to replace
Replace it every 50,000 miles and then you won't have to guess.