Shaded pole motors like fans have can be controlled to some extent by a dimmer switch, but it's very inefficient and unreliabile, and will greatly reduce the life of the motor because they will run hot. The motor will also hum terribly on lower speeds.
I won't get into technical details, but dimmer switches don't actually adjust voltage, they adjust duty cycle. Regardless, these motors by design have their speed set more by line frequency than by voltage. A variable frequency drive (VFD) is the right control for the job, but they are crazy expensive. As a compromise, use a speed control designed for ceiling fans. They'll work okay and shouldn't harm the motor. They also shouldn't cause hum.
Powerheads use permanent magnet motors, and can only be speed controlled by varying the frequency (see VFD above). Even ceiling fan controls will not work. You are far better off finding a way to constrict the output of the powerhead to reduce the water flow. Or, use smaller powerheads, or powerheads that are designed to be speed controlled, such as the vortechs or Tunze type deals. The latter powerheads are specifically designed to be speed controlled and use a special motor. That is why the potentiometer controls work with them; they're built for it.
Keep in mind, when I talk about fans, I'm talking about AC fans like the clip type fans, not the small computer-type fans that many folks use - these are DC units that can be controlled more easily. In fact, there are manufacturers that make speed controls specifically designed to control these types of fans, most computer stores that carry "modding" supplies have them. You can also get them online at places like newegg, frozenCPU, etc.
