A picture would help
1st, the Pump & the radiator fan plug into the top left powerboard, in the extreme far left headers, labeled as CPU_LC_Pump1 (black 5pin single-row) & Sys_Fan2 (basic 4pin PWM white)

Instead of your term CPU Fan, could we also use my term Rad-Fan? It's up there in the cooling caddy behind the radiator? If so, could we also call it Sys_Fan, or System Fan? Sys-Rad-Fan, lol
I've seen Dell server fans that have a 5pin on it (like the pump motor has), like
these ... otherwise W550R is the correct pump but nearly 100% of the time it's coupled to the 120x38mm
PP749 4pin PWM fan. Maybe the prior owner just tossed any old fan in there, thoughts?

I don't know what fan u have, but a qwick google of
Dell 5pin fan diagram nets some results above. Looks like the wires are mismatched & would need to be changed over to the 4pin PWM pinout (see below). Attack the back of the 5pin, use a thumbtack or needle or precision tool, push down (compress) on any slot-pin-arms, back pin out, re-set the pin arm to 45degress-ish, push pin back into new slot til it cliks in place, ez-peezy. Do that til u have a PWM pinout, but with one exception: the pinout has to align with the powerboard white header relative to the black 5pin positive top latch, see photos below to make a new (mirror-image) pinout


Since you want to
stay away from the black VGA headers, which will make your fan run at 100%, instead
simply push your 5pin over the Sys_Fan2 header as above so the latch touches the white header tab which helps them stay together
***Note that a cooler pump 5pin is shown since this is my photo I use to help people plug the pump into a basic 4pin pwm header. I did however paint in the black-red-white-blue boxes & circles for you to use as a guide if needed
***Bottom line: convert your 5pin over -or- buy a PWM PP749 fan