Pump speed goes down when adding a pump to loop

[BareFinn]

When I use my Corsair XD5 pump/res on full speed, my Alphacool Eiswolf GPU pump goes down from 2600 rpm to 2200 rpm.

Anyone know why?

(Yes, I have double checked that I have connected In and Out correctly on GPU.)

If anyone wonders why I have both a XD5 and a Eiswolf its because I have just upgraded from an Alphacool Eisbaer and Eiswolf loop to Corsair Hydro X XD5 and XC7. And 3 triple Hardware Labs rads. I kept the Eiswolf GPU block because I wont upgrade GPU Block before I upgrade my GPU.
I thought that my GPU pump was dead because of the known death rattle they eventually go into, but after my new loop was ready I tested it for ****s and giggles and found out that it was still working fine (probably a huge air bubble).

Almighty Google did not give me any answers.

 

[trents]

Are the pumps in he same loop? An attached picture of the routing of the system might be helpful.

 

[BareFinn]

They are in the same loop yes.

XD5 Pump res --> 360 Rad --> 360 Rad --> CPU block --> GPU Block/Pump --> 360 Rad --> (XD5 Pump) etc.

I can post a picture of it later if needed.

Edit:
Every fan and pump is connected to a Corsair Commander Pro. Could it be that it reports wrong when the stronger XD5 pump goes full speed?

 

[EarthDog]

It could be confused, the controller, yeah. How many amps can it output for the pumps?

 

[BareFinn]

6 pwm ports. 1A per port but max 4.5A total. But my fans use 0.72A total plus GPU pump 4W (about 0.33A) and the XD5 gets power from PSU molex and only use the port on the controller for pwm control. So there's plenty power.

 

[trents]

Could this be due to some kind of back pressure due to the different flow outputs of the two pumps?

 

[BareFinn]

70L/h at 0.85 m head pressure vs 800 L/h at 2.1 m head pressure is world apart. But I don't think it's detrimental to the water flow because when I put it on it helped with pushing some air bubbles.