I live in central Florida, it freezes here sometimes.
Bertha is a tropical fish, and she's a giant. She outgrew my 75 gallon tank, so I had to put her in something bigger. I don't have room for a tank big enough for her in the house, and I don't even know if I'd need to reinforce the floor for such a big thing anyway. lol So, I set up a stock tank outside, it's 300 gallons (about 1,000 liters, I think). I didn't bury it because there's a bit of concrete path under the tank, and also digging is hard. Now I wish I had done the work, but I have to make do now. There's hundreds of pounds of sand in the pond, and I am getting too old for this shit! I used garden netting and hay to wrap around the pond, on the north and west side the hay wall is probably 2 feet thick, on the south and east it was much harder to get the hay to stack up high enough, so I used all my bricks and paving stones to build a wall. I'm also going to cover the pond with a tarp to keep heat from escaping. I am trying to decide if it will be best to keep the bog running with the heater, or if I should switch to an air stone when it is really cold. It is very difficult to find other people doing anything like this, but I did find one guy in Alabama who has an 800 gallon container pond and he used 4 1200 watt heaters and a tarp, and he didn't even insulate the outside. He was able to keep tropical temperatures, and it gets much colder in Alabama than it does here.
(my mother thinks I am insane)