Oh, in that case:
I have done both coolant and CPU temp control. Of the two I prefer coolant temp for a few reasons:
- Monitoring based on coolant temp allows the fans to ramp up when your GPU is working but not your CPU. Things such as folding on my GPU only or many non-CPU intensive games would see the coolant temperatures rise considerably.
- Control based on coolant temps is smoother - no sudden fan spikes as your CPU load fluctuates
- The fans keep working after the load is removed until the coolant is down to a lower temp
- Your fans are cooling the water, not the CPU. Makes more sense to control them based off that
If you can't monitor the coolant then it is best to monitor your CPU rather than GPU. The GPUs are less sensitive to heat than the CPU and they tend to run a lot cooler than CPUs under water anyway.