Planted Tank Obsessed
Join Date: Mar 2009
Location: Los Angeles, Ca
I had a 20 gallon for a couple of years with no heater. I would say that is about the biggest tank you can get away with not having heater. This is in California BTW.
I had many types of Gourami and tetras. If needed, there are fish that tolerate slightly colder water like Barbs.
Like they mentioned above, It's all about your temp. Monitor your temperature at different times in the day (first thing in the morning, midday and night) and you can get an idea of temp range.