With a open loop you should be using the water temp as your basis for fan control, or even better use the delta between water temp and ambient.
You can do this several ways but the best method is with either a Aquacomputer Quadro or a Aquacomputer Aquaero 6XT.

Thanks for the heads up on using water temp as a source for fan speed adjustment, the Quadro is now something I am interested in, will it accept two temperature inputs for CPU/GPU loop temp or do I need two units, the marketing blurb isn't very clear, also where would be the optimal point to take temps?
 
Thank you for you answers and interest in the topic.

Personally I dislike the ramping and falling fanspeed sounds, so I just set my fans for my loop to the max speed for the audio level I am comfortable with and hard lock it there. No need to check Temps then either.

Papazmurf
I just got this board since I ended upgraded from my 7920x to a 10940x on my main rig.  I'm planning to custom loop this like I did on my other brand board.  I see that some users here are recommending Aquasuite to manage fan speed.  Do EVGA boards not offer this functionality with temp headers so you don't have to buy something like Aquasuite?

No motherboard on the market offers what the Aquaero/Quadro/Aquasuite offer.  
I can sit here and try to explain how it all works but it would go over most people's heads just the amount of control you have.

On my Dark I set the VRM fans too off when there is no real load (temp on the VRM) and ramp up just to keep run time low on those little buggers
The PCH / M.2 fan I thought I was able to set that lower - have not had an issue with noise from that one
Only real issue with noise on my system was failing Varda fans I bought from EK when I got the rest of my loop parts - have since switch out the fans to Noctua