I'd recommend against relying on the UPS, they aren't designed to provide hours of current, and battery degrades 10% per year.
I've got a few of them on computers at work and we are on a 5 yr replacement plan. Basically after 5 yrs, they can only run a computer for 5 minutes, vs 45 minutes when brand new.
We lose power in the shop a couple times a year, and I'm not convinced they are reliable enough to keep my tank running for very long. I WILL say they are the most easy to setup and most widely available option tho, and definitely better than nothing
A sealed deep cell battery, inverter, and solar charger will cost about the same but last longer and give you more running time.