No clue on the fish tank, but you can log power consumption and estimate the amount of stored power you'll need. For my CPAP, I bought a 100AH LiFePO4 battery (similar to a car battery, but vastly more capacity), a standalone charger for the battery, a tiny DC-DC adapter for an embedded computer, a ...

If you’re flying, you can’t bring something like this, but my solution to the power outages in my town/region is a 100Ah LiFePO4 12.8V battery, a car adapter for the CPAP, and a charger. I manage them all with an embedded computer that also runs off the battery. It’s about 20lbs (comparable capacity...