Running a 12V PWM Water pump on a 5V fan output with tacho input
If you need to use low side switch for the fan, the tacho signal will not go to 0 V. There are basically two choices.


Change to high side switch, or 
Add a comparator with open drain output to monitor the tacho.


I recommend the first choice.

There are also a couple of other issues with your design. P-FET doesn't work well the way you have connected it. Use N-FET, or change to high side switch where you can use that P-FET.

Also, the fan is an inductive load. You should add a diode parallel to the fan to offer a path for the current when the FET is not conduting.

The tacho signals are usually open drain, so you need to have a pull-up resistor for it.