I have almost all my tanks unheated year round, in Canada. This morning, the temperature of the water was 24 after a very unseasonal cool night. It's been at 25-26 for a few weeks. In January, it may be closer to 20 on lower racks, and 22 on higher.
Most people will heat their homes to around 19. I have my tanks in a dedicated space in an outbuilding, and when I built the space, I invested in serious insulation. The room is heated by a combination of tank lights, linear piston air pumps and a wintertime dehumidifier. I installed electric heaters, but they rarely come on. If it gets below minus 22 outside, they operate.
I only keep one annual species, Nothobranchius palmqvisti. They spawn in winter or summer. If one were to use your idea, you would need a warm place to incubate the eggs for Nothos. It's been many years since I bred any South American annuals, and warmth for the eggs was crucial for the ones I had.
I have a few non annual killifish species that do the same, though my wild caught Benin killies, from very warm water, only spawn in summer. They grow all winter at the slightly cooler temperatures. I only caught them this Spring, but they're species I've kept before and expect to follow that pattern.
I keep Characins unheated in most cases, but use heaters in tanks to breed them. Their breeding season tends to be around December, so things like cardinals need extra warmth. My dwarf Cichlids tend to be from forested regions of Gabon, where we measured the water at 22. I do use heaters in 2 Congo tanks and two Benin set ups.
We can go totally unheated, if the room the fish are in reasonably warm in winter. I used to live in a harder water region and had almost all livebearers from southern North America, down to Benin and Guatemala. They did very well unheated, at 20. The key to doing this is to read. You have to research species by species to make certain you're in the natural temperature range, even if you are on the lower end of it in winter. You can't try to force fish into temperature niches that aren't naturally in range for them.
"Cold" water fish are misnamed. Around here, I can walk on the water for much of the year, no miracles needed. 4 or 5 degree water out of the tap isn't strange. When we talk of cold water fish at 18, I'll call that cold for tropical.
