For me problem isn't the "slower" pump. It's working on reported 800 RPM (I have flow indicator) and I'm running it most of the time at 1100 RPM. The second one on "minimal power" is significantly louder than the first one. The second pump wasn't used before, may it be faulty ?

I just finished building my two loop system and I am having trouble with the speed control on one of my pumps. I have two pumps: "Aqua-Computer D5 pump motor PWM - 41105" connected to "Aqua-Computer aquaero 6 LT USB fan controller - 53234". The first one works fine and allows to regulate the speed to about 35% of the PWM signal which gives about 800 RPM. The second one, however, at the level of 1% PWM still rotates at 1810 RPM, and at the level of 50% PWM signal sets the speed to 3699, while the...