Thanks that’s an excellent idea a heater set at 20 just in caseUnfortunately many fish are missold or have poor guidance by the stores that sell them, so you're not alone there by any means. This is why the photo of the fish was so essential since Cories are one fish that frequently get misidentified and that usually impacts the living conditions required for that fish.
It is very difficult sometimes not to get caught in the trap of misinformation where fishkeeping is concerned. I understand the dilemma of the heating costs but if you use a heater in the temperate aquarium, set at 20 or 21 degrees that will enable the Cories to thrive without costing the earth to run in electricity....lower temperature and a well insulated home will mean the heater needing to come on will be less but it will be there should the temperature drop for any reason, it would be a backstop rather than an essential for keeping things tropical enabling the fish to stay healthy and as reassurance for you that they are not getting too cold.
When I had temperate fish I always had a heater set at around the 20 degrees just incase of problems such as a cold snap...the heater was always there should it be needed