I think you mean without a heater but if you dont the temp for
the average tropical fish is 19 degrees celsius to 30 degrees
celsius. coldwater fish will survive with a heater if its at a low
temp and so can tropical with a heater at a low temp.
There are millions of tropical fish in the tropical oceans. But
they are all marine fish not freshwater fish. Freshwater tropicals
can not live in saltwater.