Pump on its way out?

[Cold_And_Miserable]

I've been fiddling with my water system and I've noticed the D5 pump is performing very poorly compared to a cheap pond pump.
The D5 cools the EK supreme and the pond pump cools the MCW60.
The pond pump is flowing strongly, the D5 flow is moderate in comparison.

I opened the D5 earlier and was wondering if I hadn't put it back together properly. When you remove it the pump breaks into two parts, are these meant to hold together or rely entirely on the screwthing?
Theres no way I could push them together anyway as I already tried.

 

[w3bbo]

The reason for the low flow is most likely the EK supreme. It's a very restrictive block.

 

[DampCat]

Swap the pumps around and check if it's still performing poorly or if it picks up a bit.