aquaclear 20 how does it work

Yes, you want to make sure that you fill the filter up with water before starting it up, otherwise you risk burning out the motor. You also want to make sure that the water level in the tank isn't too far down either, otherwise it won't be able to pull the water up from the tank and into the filter.

The way the filter works is by moving water away from the intake tube. This creates a vacuum forcing the tank water up the tube and into the filter. With out water in the filter when you turn it on, it won't make the vacuum.