Please help me with my stir plate [video]

[TexasStrangeBrew]

So I have a pc fan wired to a cell phone batter. 12v power supply was way too fast and 5v was too slow. The magnets are strong enough to be magnetized through the beaker; at 5v it would just stop spinning and stick to the stir bar when it got close enough. In the video I have a 9v power supply which is still too fast. I had a 25ohm potentiometer hooked up but the amount of resistance wasn't enough and didnt slow the fan down much at all. In the video I have a 50ohm potentiometer and it doesn't taper at all, just turns it on and off. What could I adjust to make this thing work. What I thought would be an easy project has turned somewhat frustrating. Any help would be appreciated and a I'd be more than happy to mail you a homebrew in return

Thanks!