Not all Tropical fish revel in water at a constant high ambient temperature. Infact our fishes come from far-flung places with widely varying temperatures. Therefore it is not surprising that some of them can stand low temperatures of 65F-70F while others prefer much higher temperatures of 78F-82F. In small bodies of water the night temperature can fall 20F or more below that reached in the noon day sun. At any given time there may also be a similar large gradient of temperature in the pool,the top layers being much warmer than the bottom ones. So it is important to check on the fishes being placed in the same tank.