Trickle filter/ sump perhaps?
They sit underneath the main tank and are connected to it by hoses/ pipes. They increase the total water volume of the tank and you can put heaters, filter material or whatever you want in them. A lot of people use them on marine tanks but they also work just as well on freshwater tank. Maybe google trickle filters and look at images of them.
Some tanks have a trickle filter built into the back of them.
Do not buy a tank with the trickle filter built into the hood because they are a pain to clean and work on.