To have a water proof spinning shaft, Oral B electric rechargeable tooth brush might work?!
Looking awesome! You need to hook up a pedal control for the motor. Like the dentist has.
If you could get the thinnest siphon tube you have there to fit snugly over the end of the brush assembly- with only the brush exposed, then you'd need an elbow (attached to the siphone tube) and not a y to siphon off water right at the brush/algea point of contact..
Fear or no fear..
I think you should really try a sea hare. They can go through a tank like yours in no time and completely eradicate hair algea.
Just have to get a healthy one. They are quite amazing.