Hi pjf,

I searched Aquacomputer webshop for model 21589 and came up blank, so it may not be sold anymore, but that should not matter.

The waterblock should have 2 threaded holes for fittings (in & out).
From what I saw on other Aquacomputer waterblocks that were in the shop, the fittings will have either G 1/8 or G 1/4.thread.
You would simply get fittings that fit the waterblock thread, with tube connections size to fit the tube you want to use.

i.e. if the thread is g1/4 and the tubing you want to use is 3/8" ID, you would get something like THIS.
Just screw the G1/4 thread into the block and secure the tube on the barb.

Then do the same for your pump, selecting a fitting to match the thread size of your pump, and a barb size to match your tube (3/8" ID in example above).

The Germans tend to use smaller tubing size than the Yanks... the effectiveness is not really affected by tube size.

Euro tube sizes are stated in mm & U.S. sizes in inch fractions, but you can convert.
i.e. (Euro) 13/10mm G 1/4 = 1/2" OD by 3/8" ID G 1/4 (Yank)

Edit: Your Koolance Pump/Reservoir has 'rear G 1/4" threaded fitting sockets'.