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~snip~
this is a very good idea.
could i also connect the fans to my breakout using a 4 pin - 6 pci-e adapter (if there is one)?
i have now also ordered an usb meter

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But I don't understand why wouldn't you just get a DC fan and plug it into the GekkoScience fan output
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i didn't think of that

i will do it this way and connect my 140x140mm noctua fans to the gekko hub via the usb port provided.
As long as they're 3-pin fans, they will plug straight into the hub's fan output - that will run them at full speed. To run at roughly half speed (5V instead of 12V) you can solder them to the end of a USB cable and plug them into USB. But you could also use a Noctua low-noise adapter instead, that is usually shipped with them.
Keep in mind if they're 4-pin, you will need a PWM fan controller of any kind.