Thanks to everybody on here. I would highly suggest for people who don't have time to make a speed controller, just try out different voltage adapters with your fan. I finally settled on one (6V) and spins a 2 inch stir bar in both my flasks perfectly (1000ml and 2000ml). I made the power supply quick disconnect in case it ever goes bad I can simply plug in another one, and easy on/off switch with LED indicator. I had all the parts sitting around except the box, so total cost was $5. Thanks for all yalls help!