Magnetic Stirrer

Update 04/08/2020: I finally got around to installing a PWM for speed control. It works as expected, and I'm very happy with it. So far I have been using it for mixing Calcium and Alkalinity solutions for my marine aquarium as well as for starting yeast cultures for home brewing. A heated plate would probably be better but I haven't had any problems with just using water at 30 C and letting the stirrer do the rest.

Assembly video: https://youtu.be/ugCic-LoJRo

Magnetic stirrer v1
It works using a 12V 120mm/ 25mm PC fan to which I glued the “Magnet Disk” and inserted two magnets into. Magnets are 8mm wide and 10mm tall.
At the moment I’m running it of a lab power supply at 5V as the fan spins too fast at 12V and my stir bar can’t keep up with it.
I have included 3 different sized disks with my design as the distance between the two magnets used depends on the stir bar size. I am using disk 3 with the magnets inserted in the two closest holes and it works with stir bars betwee