The off the shelf UPS systems you can buy for computers etc. really don’t cut it for aquariums. You want pure sine wave current and big battery capacity which can be really expensive. You also want to use your batteries to increase their life or when you need them they will be dead. They need to be regularly used and charged to stay healthy. Drained to 50% then charged back up is best.
For my UPS system on my aquariums I went with a 24 volt pure sine wave 3000 watt inverter and 60 amp solar charge controller all in 1 unit. This unit was connected to 12 2 volt cell 330 amp hour batteries, 6 250 watt solar panels and the mains at my house. This gives me nearly 8 kilowatt hours of battery storage, 1500 watts of solar panels for charging and running my tanks during the day and a connection to the mains for charging if the sun doesn’t come out.
My tanks are currently drawing approx. 250 watts, so not including the charging with the solar panels during the day or the mains I can run my tanks for 32 hours off the batteries alone. Each tank has 2 return pumps and a circulation pump, I run 1 return and the circulation pump off the ups and as a safe guard in case the inverter fails I run 1 return pump and the heater off the mains.
The whole setup is plug and play from a local shop and the batteries are second hand but have a 20 year expected life. I installed it all myself and made a box for everything to go into so it is neat and tidy. It has reduced the power bill a little but the most important function of the unit is for when the power goes out. The whole setup costed me $3k AUD but it is all automatic during a power outage, there is no need to be at home and plug in the generator. During the day my tanks run off solar, during the night they run off the batteries. You plug your computer into the unit, set it up to how you want it with voltage cut offs etc. and away it goes. Easy money.