I did notice you can decrease the voltage on the controller tab of the aquaero but it decreases the voltages to all the fan inputs simultaneously.
Yes, it will adjust the output of the fan port(s) you assign to that controller, and consequently all fans attached to that fan port.
If you wish to run some of the fans at a different RPM, they should be on a separate fan port, and then make a 2nd controller for that fan port.
Are you running 3 pin voltage control fans on the OCTO?
If so, you will not be able to control them, they will run 100% all the time.