This may sound strange to some... but it works wonders for me. I don't use this setup for a FISH TANK... though it could be used for that. I use it for my Computers.
I have an older UPS (APC SmartUPS 1400) that back in it's days, it would power my computers for about an hour on battery. As batteries go, they died. So instead of replacing them with the small batteries they come with (They are 12 volt batteries run in parrellel - 24 volt system), I rigged it up with 2 car batteries (ventless batteries of course). Optima makes GREAT marine batteries that are ventless (meaning they do not discharge poisonous gases as some batteries do).
Now, when power goes out. I can run 2 computers + 2 monitors (probably about 1000 watts) for at least 14 or 15 hours.
So I would imaging something like that would work for an aquarium as well.
You should be able to pick up an old used UPS from a computer show or something... and these batteries are not cheap. Like $110 apeice.
Here, I took a pic for you.
Hope that gives you some ideas.
![](https://forums.saltwaterfish.com/image/noimage/true/width/525/height/525/flags/LL)