PWM water pump flow control - should I use capacitor for MOSFET? I want to control flow level on 12V water pump with PWM pin, I'm using IRF520N MOSFET for that (and dedicated 12V power supply).

Should I use some capacitor on MOSFET output to "smoothen" voltage? And what kind?
I think you're talking about decoupling caps they are used to smoothen out the spikes and ups and downs in the power. But you're trying to control the flow level using pwm and if you add the Caps that would smoothen the pwm signal!