First pump slowing down when the second is turned on?

I've been running two DDCs for pretty much forever.

Until now I've had them connected one next to the other with a double nipple, but I've recently taken down the whole loop for maintenance (had to repair one of the pumps), added a res (wasn't using one other than temporarily to fill the loop through a T-line), and thought I'd move the second pump further up in the loop for convenience. There's still no air in the loop, the res is used as a large transparant tube for aesthetic purposes, as I switched to semirigid rubber tubing.

Loop is as follows:
RES->DDC1->VGA->RAD1->CPU->DDC2->RAD2->RAD3->RES

I noticed that when I turn on the second pump (which is positioned after the CPU block, right before the second rad), the first one goes from 4600 to 4300rpm. Can't tell the RPM on pump 2 because I was out of wire and didn't solder the tach on the replacement board.

Why is that? When I used to run them right next to each other, turning the second pump on would increase the speed of both.

Problem solved: I installed the wrong pump by accident, one that I haven't upgraded the PCB of. As the stock pump runs at lower RPM, it slows the first one down to the same speed.
I'm just going to run a single pump until I have some time to kill and replace the other.

Apparently I was wrong, someone on overclock.net pointed out that pump RPMs are not linear depending on pressure and flowrate:

https://www.overclock.net/forum/atta...p;d=1545348931

So basically as the second pump increases pressure, and as the first pump has to produce less of it, as it is now experiencing less restriction, the loop's point along the curve moves to one with higher flow, that also happens to be at lower RPM.

The more you know.