Hi David Levy
you have a few options open to you to control the speed of your two 230mm fans
option 1 - a fan controller
option 2 - a molex to fan speed reducer to 12V (flat out ) to 7v ( slower and less noise ) 5v ( slower still and less noise again )
option 3 - a fan pcb board - a pcb with multiple fan headers on it at various voltages usually is powered from a 4pin molex
option 4 - a single fan speed reducer for each fan a reducer that plugs into a 4 pin molex and to your fan with a dial inbetween for manually adjusting the voltages
option 5 - adjusting fan speed from within the bios - setting fan speed profiles to the rpm you require from headers on your mother board
option 6 - thirdpart software like speedfan
http://www.majorgeeks.com/files/details/speedfan.html
https://www.youtube.com/watch?v=8VjdQStihsE
you should also be able to adjust your fan speeds through asrocks oc tuner