arduino: turn on-off a 12V cpu fan with optocoupler
With 10 mA LED current, the 4N25's current transfer ratio is 20% minimum - that would give 2 mA throught the output transistor.

The specs for the fan call for 1 - 2 Amps - there is no way the 4N25 can directly drive the fan.

You need to have the opto drive a FET that can handle the 2 Amps the fan may want on starting.