I've got four "tropical" tanks on the go these days, used to be six. Only the tank in the poorly insulated garage had a heater running typically Nov-Apr at 20C, all the indoor tanks were and still are heaterless unless their water drops under 18C in a bitter cold spell.
I'm talking various African species (Butterfly Fish, Leopard Bushfish, various Synodontis, Distichodus, Pareutropius etc.) plus Dwarf Chain Loaches, L204 plec and a Rubbernose Plec.
A lot of these fish have been with me for close to 12 years and this is how I've always run my tanks, their water temp changes with the seaons, typically ~20C in winter to ~29C in crazy summer heatwaves.