Invented for who knows what but popularized to consumers in the 1980s and 1990s for computers, these are a roughly shoebox size metal box with a heavy sealed-lead-acid ("SLA") battery inside. You plug it into the wall and now you can plug things into it. The batteries inside are charged from the wall and when you plug things in you are drawing power from the batteries not the plug. If you unplug it from the wall plug, it still powers the things you plugged in to it.
Always get the biggest one you can.
Also called a battery backup power source, they're pretty much essential if you want to keep the heater (if you have one) and pump/filter or lights going when there's no power at the wall plug.
Pro tip: hang solar panels nearby, charge the batteries with this rather than the wall plug.
These things come in all sizes. All computer stores have them. Small ones are about the size of a shoe box, the largest ones look like a overside PC tower and have two car batteries (actually car-sized) batteries inside but are 6V each and the "deep cycle" kind, the ones used for boat trolling motors and electric lawn tractors - GE made these in upstate New York beginning in the 1930s. still sold today, they were the ones used in the first EV cars in the 1970s). The small ones are SLA (Sealed Lead Acid) while the 6V deep cycle ones are not, and you have to be aware of and deal with Hydrogen fumes when they're charging. Just vent it.
If you're handy you can build this all yourself, connect the solar panels to a charge controller to the batteries. Now get a 1 or 2kw "sine wave" inverter - not a saw tooth one, these will only work with some devices), plug things into that, done.
If you just buy one in a box from whatever-mart, it'll work. The small ones should run a pump or filter for a few hours. Light use more. Heaters use the most and of course you don't want a heater running without water movement from a pump or filter of some sort.
Roughly speaking, you should get a good portion of a day with a small one and a power filter. Air pumps, maybe 2-3 days. Lights, good ones maybe half a day. A tad more for heaters. Of course you can use multiple or bigger units.
A large or commercial setup might be half a dozen batteries wired up in an array, taking power from the wall or solar panels or wind or water turbines.
A very large setup would use a telco battery. These are one ton batteries with lead plates so thick you don't need a charge controller, you can't overcharge them. They'll run a room of dryers and the only limit really is how much energy you have to charge it with and you can use a lot of panels with these. Of course you don't have to, you can call a charging service and they'll "fill it up" for you if you could charge it from the wall.
That being said - you certainly do not need one but it is really nice to have.
