If you want one to work with your motors, you will need one that uses a true sine wave power inverter. This is the trick to running the motors smoothly. The second best is a stepped wave inverter, or worst case (el cheapo) is the square wave. Avoid the square wave like the plaque, it works for some things (like computers) but definitely not for aquarium motors.
The smooth curve is the true sinewave. This is represents how the polarity in our 120V wall outlets switches polarity at 60Hz. This is what our fishtank equipment was originaly designed to run with, so the UPS should match this.