Thanks for the comments guys, the more knowledge shared the better!
Today was a rather disappointing day for me, while I tried to setup the Growrilla RDWC. Watching the setup instruction movie I noticed several things (holes in the bucket where not there), one of them was how to connect the waterpump.
This waterpump Hailea HX-2500, has on the outlet a second smaller outlet to act as a venturi. The hole for this was also missing. Luckely I anticipeted this from early on this project and bought some tools (like a drill). You can see the little venturi outlet pointing backwards.
Since I also ordered al rubber parts double, I had enough material to make some holes and seal them off with a proper rubber ring.
I added water to the system, let it sit for a while to check for leaks. I did perform this test in the bathroom, in cases there would be any leaks. Luckily no leaks, but unfortunately a real fountain spraying from aside the bucket. As soon as I powered on the system the water started to spray from out of venturi ventile

I also noticed not very much movement, like the pump was taking water from the rservoir it sits in, instead of the buckets.
So this made the pump useless for this project, with the venturi outlet water gets pushed back into the reservoir...
Below two pictures of the Hailea HX2500, with the odd piece of hard plastic between it, with twice a 90 degree corner in it.
Since I can't do much without an waterpump, I already had bought a spare one (bought allmost everything twice, from lighting to heaters etc. in case of faillure); (Alien Jetstream) and it did not fit because of the hard plastic parts to connect. Since a had enough tubing a cut a piece off and replaced the plastic part. This pump has no venturi, but at least I can transfer the plants now.
Which is hardly needed since especially the roots are growing really fast.