APC's Back-UPS line is designed for use with Computer-type loads only. They are not designed to be used with motor loads such as fish filters, air conditioning units, space heaters, vacuum cleaners or any other machinery.
One potential issue is the inrush current draw for a motor. It can easily overload a Back-Ups UPS model. Secondly, these models of UPSs output a Stepped-Approximated sine wave when on battery. This waveshape, while ideal for computer-type equipment, is not particularly compatible with most motor loads. The result may be that motors run very slowly, erratically, or not at all when the UPS is on battery.