In my experience with G. gymnogenys 'Arroyo Yerbalito', I have found them extremely tolerant of a wide range of temperatures. Last summer I put about 30 adults into my outdoor pond. In the heat of the summer they thrived (water temp 80+). As the water temperature gradually
(and that's the key) dropped going into the fall, they showed no signs of a problem. I finally removed them from the pond on the 2nd of November, along with ~30 Lepomis megalotis (Kansas longear) and 16 goldfish. There was a thin skin of ice (1/4") on the pond when I drained it and caught the fish. All the Gymnos were fine.... showing incredible color and condition as were the other inhabitants. Throughout October , the pond temperatures were in the mid to low 40's and when I pulled the fish, it was 36oF water temp, air temperature 28 oF.. They suffered no ill effects from temperatures that were much lower than it gets in their native Uruguay. The 5" males were displaying prominent nuchal humps. It seems to me that the range of temperatures that this species can withstand is greater than in their natural environment. I'm not saying that these extremes should be provided, but a cool period is strongly advised. Maintaining them indoors at 68-75 oF is perfect. Here are a few pics of he Gymnos, one female seen in the net on that cold day, some longears "glowing in the fall sun, and then swimming in their over winter tank (180g) along with their pond mates. Hope this adds another perspective on temperature and wellbeing.