that's why i'm looking for ideas haha, it's hard modewould be so so much easier to give you stocking suggestions if it was heated soft water. you rarely see small cold water hard water set ups with tannins. in general there arent many cold water fish. and theres very few small cold water fish. and most don't have much colour. whereas I could go on forever with stocking suggestions for a small tropical soft water tank.
We will consider a heater though
I know a couple of endler breeders that keep them without heater, in nature they live in very warm water but i think they can adapt well to different parameters, they have been found in cold or acidic environments in nature... obviously you can't keep them at 18°C forever, but some seasonal changes in temperature are reportedly good for fish healthOP states it goes down t o 18°C/64F°. Endlers are very much Tropical temperature fish.
Another option would be just picking up fish from local irrigation canals (that dry out in winter) and keep them for a bit before releasing them in the summer
EDIT: i may have remembered incorrectly about endlers being found as invasive species in temperate or softwater, that applies only to guppies as far as info on the internet goes