I designed one years ago because there is an obvious need, but the prototype was going to cost $250.000 so I bought a generator instead.
Search "portable power bank", "portable power pack". There are several on the market. They don't have huge output, but would run a fish tank ok. 100-200 watts seems a common size, but there are bigger ones too. You would have to figure out your electrical needs vs. power output of the unit to be sure it will work for you.
I've never used one so I can't speak from experience, but at first glance it seems like an option.