No worries. If you're going to use a consumer grade model I would plug your aquarium filter, air pump, and lighting into surge-only outlets. AC Motors won't handle the step approximated sine-wave well at all, and I'm pretty sure it's a bad thing for fluorescent ballasts too.
This website has a brief explanation
, it's in the context of solar panel inverters but the same conclusions apply to UPS'es as well:
Some electronic devices may pick up inverter noise while operating with modified sine waveform. Using fluorescent lighting can be problematic when using modified sine wave inverters. Most of the equipment on the market is designed for use with sine waves. Some appliances, such as microwaves, drills, clocks or speed motors will not produce full output if they don't use sine wave current, moreover they may damage the equipment. Some loads, such as light dimmers will not work without sine wave at all.
It's safe to say any electronic device that requires sensitive calibration can only be used with pure sine wave inverters. For many electronic devices that don't require sensitive calibration, modified sine wave inverters are a more cost-effective option.
Personally, I don't like to use the cheaper UPS'es for anything other than resistance loads, like incandescent or halogen lighting. I know they are the only option for people on a budget and theoretically
they shouldn't damage your PC, I'm just more comfortable feeding my equipment the waveform that it was designed to operate with.
(I say theoretically
because the power supply in your PC should isolate the motherboard/cpu/other components from inverter noise. It does increase the burden on the PSU though, and if you have one with temperature sensors you'll notice that the PSU temperature increases while on UPS power. If you're only on it long enough to shut down safely I wouldn't worry about it.)